From your question I see you want to reset the password, unlike you have forgotten what your root password is and planning to take the bigger route here.
If you still have the root password working, you can just login to console and reset it to a new one. Use the passwd cmd and it will prompt you to set a new one.
eg:
root@GTM-A passwd
Enter Password:
Re-Enter Password:
For updating the password, a reboot is not required.
In case you have forgotten your password and you are unable to get into the console. Then you have to follow the single user mode which does requires a reboot.
Since you have 2 GTM's which would be in Sync Group, there would not be an Active/Standby concept. You can take down 1 GTM have the pwd reset, in that moment the B GTM would be proceessing the traffic. There will not be any downtime. Then once your A GTM comes online and validated all good, proceed doing it on the B GTM. Hope it clears.