Increment/Decrement Number using Up/Down Key on Keypad C

I am new to keypad programming. So please bear with me. I have Variable say, ABC = 1234.5 which is being displayed on LCD properly. But I want to increment/decrement this number individually. For eg when a key is pressed the cursor will move to ABC = **1**234.5 this position and on pressing up\down key the value will increase/decrease. Similarly for all the other values. And it will be displayed on LCD.