for the challenges go in lobby use the hack and when you kill someone then (wich you need in order to get a amount higher) then its done to use it again go to lobby and pres the button again get youre challenge kills and its again upgraded
You have to perform the corresponding action that would normally add to the count - e.g. for Scavenger, loot ammo; for Scrambler, get a close range kill; for Hardline, get a killstreak etc.
For the action to count, you would of course need to have the perk activated - e.g. to complete a Scavenger challenge, use the hack, select the perk with the class you're playing, then play that class and complete the action (loot ammo).