Why?
You already have an "up" and a "down" button (on the immediate left of your NumericUpDown Control) so all you are doing is adding redundant buttons which at best confuses users ...
But ... if you must then it's trivial: handle each new "up" and "down" button, and add or subtract 1 from
MyNumericUpDown.Value
.
I can't see your code, but it's probably something like:
MyNumericUpDown.Value += MyNumericUpDown.Value;
and it should be
MyNumericUpDown.Value += 1m;