After going through all the answers I thought i will add two simple options
If you already accessed the record using FirstOrDefault() with tracking enabled (without using .AsNoTracking() function as it will disable tracking) and updated some fields then you can simply call context.SaveChanges()
In other case either you have entity posted to server using HtppPost or you disabled tracking for some reason then you should call context.Update(entityName) before context.SaveChanges()
1st option will only update the fields you changed but 2nd option will update all the fields in the database even though none of the field values were actually updated :)