I have a datetime
You can make use of MySQL CONCAT() function.
Your update query would looks something like this,
UPDATE table_name SET date_column = CONCAT('2016-06-29 15:',MINUTE(date_column),':',SECOND(date_column)) WHERE column_id = 1
CONCAT() returns the string that results from concatenating the given arguments.
So in your case we would update date and hour by adding first argument as
2016-06-29 15:, and then use the same minute and second from the same column. And concatenate all the arguments to make the new value as you need.