Update only Time in a mysql DateTime field

Try this:

UPDATE yourtable 
SET yourcolumn = concat(date(yourcolumn), ' 21:00:00')
WHERE Id = yourid;

UPDATE myTable
SET myDateTime = ADDTIME(DATE(myDateTime), @myTimeSpan)
WHERE id = @id;

Documented on MySQl date functions MySQL docs

Well, exactly what you are asking for is not possible. The date and time components can't be updated separately, so you have to calculate the new DateTime value from the existing one so that you can replace the whole value.