If you have app access and want to update the password follow the steps in the Changing the Admin and Router Password article.
If you can no longer access the app, try using the WiFi password (during setup you had the option to use the same password for the app and for WiFi; or to use two different ones). If that doesn't work, try accessing the web UI using the WiFi password or any other password you might think is right.
If that doesn't work, you'll have to reset your AmpliFi system since there is no password recovery method. Follow the steps in How to Reset the Router to Factory Default Settings.
Wireless Network Password:
With access to the AmpliFi app, you can change the wireless network password by following the Changing the Wi-Fi Network Name and Password article.
Guest Network Password:
If you have access to the AmpliFi app, the guest network password can be removed or modified by following the steps in this article: Configuring a Guest Network.
Teleport Hardware Device: The Teleport password is the Teleport's WiFi network password you create when setting up the Teleport device. Take a look at this article to learn how: Setting Up Teleport Hardware. Unfortunately, there's no way to recover Teleport's WiFi password, you'll need to reset the Teleport device and set it up again. To reset, just press and hold the reset button on the Teleport for about 10 seconds.
Teleport App: The Teleport app does not require any password, you only need the Teleport code to make use of the VPN software. See this article for more info on how to generate a Teleport code: Setting Up Teleport with App.