update wifi password for apple tv

Your issue is not with Apple TV, but with your router settings. You are advised to set your router to not use both bands as separate networks (i.e. different SSID network names). Instead, have your router bridge (combine) both as one logical network under one name, with one Wi-Fi password, so that devices can connect to the Wi-Fi band and channel that they think is best for them. Let your router do the routing (automatically), as intended.

Recommended settings for Wi-Fi routers and access points - Apple Support

mind you passwords are case sensitive so SecretPassword is not the same as secretpassword

also some wifi routers keep 5Ghz wifi and 2.4Ghz as separate networks which can have different passwords so if you only configured one of them with your password and the appletv try to connect to the other it can give problems

also if one keep the the SSID the default then one can at times think one is connecting to ones wifi network but one is really trying to connect to a wifi network close by with a similar name

I had the same problem with my Apple TV 4K box right after my DSL modem was replaced with a new one. I figured out how to fix this while troubleshooting. In the modem settings, when I disabled the 5GHz wireless radio, devices must now connect via the 2.4GHz wireless radio. The Apple TV 4K now accepted the password and connected successfully to my WiFi network. When I disabled 2.4GHz, and enabled 5GHz the Apple TV 4K gets the "incorrect password" issue again. It seems when both 2.4GHz and 5GHz are enabled (the default setting), the Apple TV 4K was connecting via 5GHz (and getting the issue). This is really only a workaround until Apple fixes this issue with 5GHz connections and updates the software.

You cant change your routers password from the appletv you can just go into settings and leave the wifi and re enter the new password to join