how to reset encrypted backup password

You can’t reset the password, but there may be ways around that.

When you entered the backup password if you have a Mac and have Keychain enabled you were given the option of saving the password in Keychain (this is the default). So check Keychain for it.

If you haven’t erased the phone yet you can create a new backup with a new password—>About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support

If neither of those work you get unlimited guesses, so try to guess what password you might have entered back when. If you are like most people it will be a password that you reused. And to speed up the process click to change the password and you will be prompted immediately.

Note that if you do NOT encrypt your computer backup then the backup will be incomplete; it will not include health data or passwords. iCloud backups are protected by default because they require your Apple ID and password to access, so this is not an issue for iCloud backups.

You don’t. You can change it for future backups but you can not change the password for an existing encrypted backup.