<aura:attirbute name="LeadObj" type="Lead"/>
<lightning:recordEditForm objectApiName="Lead" >
    <lightning:inputField aura:id="accid" fieldName="Account__c" value="{!v.LeadObj.Account__c}" label="Attribute" onchange="{!c.myChangeAcc}"/>
</lightning:recordEditForm>



controller.js

mychangeAcc:function(component,event,helper){
var acc=component.find('accid').get('v.value');
alert(acc);
var action = component.get("c.LeadAccountInfo");

        action.setParams({ldvalue:acc});

}

apex class:

@AuraEnabled
    public static string LeadAccountInfo(Id ldvalue){
Account ac=[select id,Name from Account where Id =:ldvalue limit 1];
        return ac.Name;
}