Um, do you mean you want up/down buttons and textbox, and clicking on the up/down buttons makes the time increase/decrease? I don't quite understand but I'll proceed on the assumption that I'm right.
You just need to add code to the two buttons so that a click reads the current time from the textbox, adds or subtracts a minute (or second, or hour, whatever you decide) from the time and then puts it back in the textbox.
The DateTime functions make the actual maths trival.
Have you got any code to show us what you've done already?