Backups & restore points · Encryption and passphrases
How do I change my backup passphrase?
Short answer: Choose Change passphrase… from the set's Destination menu. It permanently deletes every existing version of that set and starts a fresh backup, so use Re-enter passphrase… instead if you only need to fix a typo.
Open the backup set's Destination menu and choose Change passphrase…. Be aware of what it costs: changing the passphrase permanently deletes every existing version of that set, because they're encrypted with the old passphrase and can't be re-opened. A new backup then starts from scratch.
That's why the button is labelled Change & delete previous and asks for confirmation. If you only need to fix a mistyped passphrase on this Mac, use Re-enter passphrase… instead — that keeps every version.