Using number strings in conjunction with buttons

hi, im a flash beginner and i need to know how to have a number string that works with an up button and a down button so eg. the text field starts on 0, the user presses the “up” button and it goes to 0.5, press it again and it goes 1.0, 1.5, 2.0 etc, and the opposite for down except it shouldn’t go past 0.

Ive set up a slider that does it easy but can’t figure out how to change the code for buttons, if anyone can help me that would be great thanks.