Some combo locks, you physically disassemble and arrange the numbers yourself. Others have a ring you twist, and still others have a slider that you hold in while changing the combo.
IMHO, your stepdaugher is a small girl, a lightweight lock is OK, but if she is an adult, I would not use a cable lock, because its SO easy for someone to break them and be off with the bike. If you want combination access, consider a Kryptonite U-lock with a combination instead.
Anyhow, I can't find any info on this Giant-branded lock, other than a picture, and it looks to me like you disassemble the lock to change the combination by sliding the numbered plastic rings off, and rearranging them.
If that doesn't help, I'd probably consider chucking the lock and purchase a similar product from Kryptonite or OnGuard.
Disclaimer: I live in a town where if people don't rip your bike off, they yank stems and forks, so my idea of security may be higher than what is needed where you reside.