I think it's probably easier to just remove the account and add it again. It's not very hard to do that, and in my experience it doesn't delete all your existing synced content or anything, it essentially just reauthenticates.
If you really want to try to manually change the password, I believe you may be able to do it by deleting the correct entry from the keyring. That might make GNOME ask you for a new password for that account, though I'm not 100% certain doing this won't mess up the whole account and force you to delete it to resolve it anyway.
If you want to risk it, you can use Seahorse to access the keyring (it's called "Passwords and Keys" in the menu), and look for the GOA entries.

They have cryptic account numbers listed on them, so you would then need to figure out which one corresponds with the account you want to change. The only place I know of where those are listed is in ~/.config/goa-1.0/accounts.conf, which will include their friendly name.
