I've changed my computer. My Time Capsule password (which I've forgotten) is in the old computer. How do I change the password to en Time Capsule in the new computer?
Assuming a normal configuration for Time Capsule and your older macOS computer:
You should be able to login, use Keychain.app to find all the passwords you had and display them in plain text. Then you can use them to to configure your new macOS computer.
If that is too brief, comment below, and I'll give you more details.