Makenna realizes there's no backing out of trying it out one more time and sighs, "Here I'll try it on myself this time."
Mia flowers her had a watches expectedly. Twisting the age dial seems to most obvious solution now, but it's not very easy to figure out the rules. When she shot Mia it was on 3 it had made her 5, so Makenna's best guess it divides or multiplies someone's age by the set number.
That kind of makes sense, though there's still no obvious way to switch it from making someone younger to older. She finds there is an unlabeled switch right above the age dial which might be what's she's looking for.
After flipping that she sets the dial to 2 and turns the gun to fire on herself. But as she pulls the trigger her thumb slips and spins the age dial. As the pink light engulfs her, Makenna gulps and hopes that won't effect her transformation.