You dont
If unit have some active talent then in description is `chargesX` or `reloadY`, if charges then unit can use that ability X times per combat, if reload then unit can use that ability every Y turns in combat - use it in first turn then (Y-1) turns later.
You can use spell gift on some unit to refresh it`s all active abilities 1st lvl spell for 1-2lvl units, 2lvl spell for 3rd lvl units, and 3rd lvl spell for 4th lvl units, 5th lvl units you cant refresh, but in the Legend there are no 5th lvl`s with charges on it`s abilities.
Have fun