From 3rd party, they are sending the data and it's getting stored in Salesforce. Now the requirement is, They are sending "Email Id" also.So i have to check whether that email id is already present in Salesforce. If it is already present then I have to fetch the data regarding that "Email id" from Salesforce and then do the update in the same data.

I have to do changes in below code of API.

@RestResource(urlMapping='/EGS_APIForOldCustomerWebTeam/*')
global class EGS_APIForOldCustomerWebTeam  {
    
    public class AccountData{
      //public Id accountId;
        public String customer_code;
        public String web_customer_id;
        public String web_customer_mapping_id;
        public String account_name;
        public String account_source;
        public String email;
        public String email_pec;
        public String vat_number;
        public String social_security_number;
        public String phone;
        public String mobile_phone;
        public String type;
      //public String billingAddress;
        public String billing_street_address;
        public String billing_city;
        public String billing_state;
        public String billing_zipcode;
        public String billing_country;
      //public String shippingAddress;
        public String shipping_street_address;
        public String shipping_city;
        public String shipping_state;
        public String shipping_zipcode;
        public String shipping_country;
        public String product_category;
        public String product_subcategory;
      //public String country;
      //public String stateProvince;
      //public String country1;
        public String first_name;
        public String last_name;
        public String software;
        public String activation_key;
        public String web_Account_Id;
        public String modules;
    }


    
    @httpPost
    global static void createAccountData(){
        try{
            RestRequest req = RestContext.request;
            String jsonString = req.requestBody.tostring();
            List<AccountData> receivedData= (List<AccountData>)System.JSON.deserialize(jsonString, List<AccountData>.class);
            
            List<Account> accountList = new List<Account>();
            List<Contact> conList=New List<Contact>();
            
            for(AccountData accountData : receivedData){
                Account acc=New Account();
                
                acc.Name=accountData.account_name;
              //acc.Customer_Code__c=accountData.customer_code;
                acc.AccountSource=accountData.account_source;
                acc.Email__c=accountData.email;
                acc.Email_PEC__c=accountData.email_pec;
                acc.VAT_number__c=accountData.vat_number;
                acc.Social_security_number__c=accountData.social_security_number;
                acc.Phone=accountData.phone;
                acc.Mobile_phone__c=accountData.mobile_phone;
                acc.Type=accountData.type;
              //acc.BillingAddress=accountData.billingAddress;
                acc.BillingStreet=accountData.billing_street_address;
                acc.BillingCity=accountData.billing_city;
                acc.BillingState=accountData.billing_state;
                acc.BillingPostalCode=accountData.billing_zipcode;
                acc.BillingCountry=accountData.billing_country;
              //acc.Country_C__c=accountData.country;
              //acc.State_province__c=accountData.stateProvince;
              //acc.ShippingAddress=accountData.shippingAddress;
                acc.ShippingStreet=accountData.shipping_street_address;
                acc.web_customer_id__c=accountData.web_customer_id;
                acc.web_customer_mapping_id__c=accountData.web_customer_mapping_id;
                acc.ShippingCity=accountData.shipping_city;
                acc.ShippingState=accountData.shipping_state;
                acc.ShippingPostalCode=accountData.shipping_zipcode;
                acc.ShippingCountry=accountData.shipping_country;
                acc.Product_Category__c=accountData.product_category;
                acc.Product_Subcategory__c=accountData.product_subcategory;
                acc.Software__c=accountData.software;
                acc.Activation_Key__c=accountData.activation_key;
                acc.Web_Account_Id__c=accountData.web_Account_Id;
                acc.Modules__c=accountData.modules;
                
              //acc.Country__c=accountData.country;
                accountList.add(acc);
                
                
            }
            if(!accountList.isEmpty()){
            insert accountList;
            }
            
            for(AccountData accountData : receivedData){
                Contact varCon = New Contact();
                varCon.FirstName = accountData.first_name;
                varCon.LastName = accountData.last_name;
                varCon.Email = accountData.email;  
                varCon.MobilePhone = accountData.mobile_phone;
                varCon.Phone = accountData.phone;
                varCon.AccountId = accountList[0].id;
                conList.add(varCon);
            
            }
            
            if(!conList.isEmpty()){
            insert conList;    
            }
            
            RestResponse res = RestContext.response;
            res.statusCode = 200;
            String webCustomerId = (accountList[0].web_customer_id__c);
            system.debug('WebCustomerId '+webCustomerId);
            String webCustomerMappingId = accountList[0].web_customer_mapping_id__c;
            String customerCode = accountList[0].Customer_code__c;
            //res.responseBody= Blob.valueOf('Web Customer Id = '+webCustomerId+',  Salesforce Account Id = '+accountList[0].id+', Web Customer Mapping Id = '+webCustomerMappingId+', Customer Code = '+customerCode);
               Map<String,String> varMap=New Map<String,String>();
            varMap.put('web_customer_id',accountList[0].web_customer_id__c);
            varMap.put('salesforce_account_id',accountList[0].id);
            varMap.put('web_customer_mapping_id',accountList[0].web_customer_mapping_id__c);
            varMap.put('customer_code',accountList[0].Customer_code__c);
            res.responseBody=Blob.valueOf(JSON.serialize(varMap));
        }catch(Exception e){
            RestResponse res = RestContext.response;
            res.statusCode = 404;
            res.responseBody = Blob.valueOf('Something went wrong! Error Message : '+e.getMessage());
        }
     }
  }
    
    
    
 
Please help me. Thank you in advance !!!!