Login your router and reset to the factory default settings. SO all configuration will be cleared out, including the guest password. Then, set a new password.
You're either:
1. the admin, in which case you can log in your router and change/reset the guest password to your liking, or
2. a guest on that network, in which case you need to ask the owner/admin for the password.
Login your router and reset to the factory default settings. SO all configuration will be cleared out, including the guest password. Then, set a new password.