OwnerID change based on round robin

I have an APEX class with an @InvocableMethod which is executing from Process builder on Lead object,

I am trying to assign Owner for new lead based on round robin arrangement, but its throwing 

String lo=fetchLeadOwner(activeLOs);
                System.debug('We chose ::'+lo+':: Based on less number of leads');
                leadDetails.OwnerId = lo; // here getting System.FinalException: Record is read-only
                update leadDetails;