formatted code :
my apex class:
@RestResource(urlMapping = '/v1/UpdateFWAReferralCode/*')
global class FWAReferralCode {

    @HttpPost
    global static Void Updatecode(Integer updatelimit) {

        RestResponse res = RestContext.response;
        if (res == null) {
            res = new RestResponse();
            RestContext.response = res;
        }
        List < Account > accid = [SELECT Id, (Select id, name from assets where name = : label.BasePlanName or name = : label.PlusPlanName) FROM Account WHERE Network__c = 'CORE'
        AND vlocity_cmt__Status__c = 'Active'
        AND Customer_Group__c != 'Free Trial'
        AND ID IN(Select Accountid from asset where name = : label.BasePlanName or name = : label.PlusPlanName) AND Id NOT IN(SELECT Account__c FROM FWA_Referral_Code__c WHERE Account__c != null) LIMIT: updatelimit];
        //List<FWA_Referral_Code__c> FWAcode = [select ID,Plan__c,Mapped_Timestamp__c,Account__c from FWA_Referral_Code__c where Account__c = NULL limit :updatelimit];
        if (accid.size() > 0) {
            system.debug(accid.size());
        }
        List < FWA_Referral_Code__c > FWAupdate = new List < FWA_Referral_Code__c > ();
        List < ID > Responselist = new List < ID > ();
        List < FWA_Referral_Code__c > FWAResponse = new List < FWA_Referral_Code__c > ();
        try {
            for (Account ac: accid) {
                FWA_Referral_Code__c fwa = [select ID, Plan__c, Mapped_Timestamp__c, Account__c from FWA_Referral_Code__c where Account__c = NULL limit 1];
                Responselist.add(fwa.id);
                fwa.account__c = ac.id;
                fwa.Mapped_Timestamp__c = DateTime.Now();
                if (ac.assets[0].name == label.BasePlanName) {
                    fwa.Plan__c = label.BasePlanName;
                    system.debug(fwa.Plan__c);
                } else if (ac.assets[0].name == label.PlusPlanName) {
                    fwa.Plan__c = label.PlusPlanName;
                    system.debug(fwa.Plan__c);
                }
                FWAupdate.add(fwa);

                update FWAupdate;
                system.debug(FWAupdate);
            }
            for (FWA_Referral_Code__c resp: [select ID, Account__c from FWA_Referral_Code__c where Id = : Responselist]) {
                FWAResponse.add(resp);
            }
            if (FWAResponse.size() > 0) {
                res.responseBody = Blob.valueOf(Json.serialize(FWAResponse));
                system.debug(res.responseBody);
                res.addHeader(Label.label_Content_Type, Label.label_application_json);
                res.statusCode = 200;
                system.debug(res.statusCode);
                return;
            }

        } catch (Exception e) {
            System.debug('Error: ' + e.getMessage());
        }
    }
}




Test class:
@isTest
Public class FWAReferralCodeTest {

    Public static testMethod void FWAReferralTest() {

        FWA_Referral_Code__c fwa = new FWA_Referral_Code__c(name = 'testing');
        Account ac = new Account(Name = 'Thanigai testfwa', vlocity_cmt__Status__c = 'Active', Network__c = 'CORE', ServiceSharingType__c = 'Group', Customer_Group__c = 'Test');
        Asset ass = new asset(name = 'visible+ plan', Accountid = ac.Id);
        insert fwa;
        insert ass;
        insert ac;
        FWA_Referral_Code__c fwa1 = new FWA_Referral_Code__c(name = 'testing1');
        Account ac1 = new Account(Name = 'Thanigai fwa', vlocity_cmt__Status__c = 'Active', Network__c = 'CORE', ServiceSharingType__c = 'Group', Customer_Group__c = 'Test');
        Asset ass1 = new asset(name = 'visible plan', Accountid = ac.Id);
        insert ass1;
        insert fwa1;
        insert ac1;
        FWA_Referral_Code__c fw = new FWA_Referral_Code__c(Account__c = ac1.id, Mapped_Timestamp__c = DateTime.Now(), Plan__c = label.BasePlanName);
        FWA_Referral_Code__c fw1 = new FWA_Referral_Code__c(Account__c = ac.id, Mapped_Timestamp__c = DateTime.Now(), Plan__c = label.PlusPlanName);
        insert fw;
        insert fw1;
        Test.startTest();
        FWAReferralCode.Updatecode(1);

        Test.stopTest();

    }
}