I'm getting Expression Cannnot be assigned when I'm trying to populate the Account field 

for(Account acc : newAcc){
            Account oldAcc = oldMap.get(acc.Id);
            for(String fieldAPI : FieldSet){
                if(acc.get(fieldAPI) != oldAcc.get(fieldAPI)){    
                    //getting error from the below field
                    acc.get(fieldAPI) = oldAcc.get(fieldAPI);
                }
             }//loop
}//end loop

 

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