Are you talking about changing a number from positive to negative? 1 or -1?
Let's decide on the number whose sign we will change.
The sentence in bold above assumes you have a list of numbers, 4, 12, -789, 22, 540, whatever, and you're going to pick one of them and change it from positive to negative or negative to positive.
Decide on, decide upon...
And if you are not obliged to use the verb "decide" you could also say it like this:
Let's choose the number whose sign we will change.
Let's settle on a number whose sign we will change.
Let's pick a number and change its sign.