how to change login credentials for single sign on

Use single sign-on with your device - Apple Support

“Change or sign out of your TV provider

If you change TV providers, you can remove your current single sign-on account information or set up a different one. Here's how:

1. On Apple TV, go to Settings﹥Account﹥TV provider.
2. Click or tap your the current TV provider.
3. Click or tap on Remove TV Provider.
4. Search for your new TV provider, then follow the steps to sign in.”

Possibly the key is to delete the old one first.

I haven’t seen any procedures for the situation of new credentials with the same TV provider. You may have to reset the device to be able to do that.

Restore your Apple TV to its factory settings - Apple Support

If after a reset, then it is not the Apple TV cache memory that does the recognition, but it is Charter Spectrum that identifies you remotely. You will have to talk to their support about this.

Could this be the announced Zero Sign-on, where you are not needed to give credentials at all? Or is the account details (e.g. TV package) wrong for you?

Charter Spectrum does note on their site to give it 72 hours for new usernames. Could that be it?

Does resetting to factory delete my apps?

Yes, the reset will erase any customization. You can re-download the apps as already purchased and restore any in-app purchases, but it is not automatic. Not sure if the One Home Screen feature is able to help with automatic restore from a cloud description of the apps on the home screen of Apple TV.