i forget password backup and want know it

Sorry, but you'll have to keep on guessing the password. If you did not write it down somewhere, it is not saved on your computer.

The only thing you could do, would be creating a new password for a following encrypted backup, as explained in this article:

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You can’t restore an encrypted backup without its password. With iOS 11 or later or iPadOS, you can make a new encrypted backup of your device by resetting the password. Here’s what to do:

1. On your device, go to Settings > General > Reset.
2. Tap Reset All Settings and enter your device passcode.
3. Follow the steps to reset your settings. This won't affect your user data or passwords, but it will reset settings like display brightness, Home screen layout, and wallpaper. It also removes your encrypted backup password.
4. Connect your device to the Finder or iTunes again and create a new encrypted backup using the steps above.

You won't be able to use previous encrypted backups, but you can use the Finder or iTunes to back up your current data and set a new backup password.

If you have a device with iOS 10 or earlier, you can't reset the password. In this case, try the following:

• If someone else set up your device, ask them for the password.
• Use an iCloud backup instead. If you don't have an iCloud backup, you can make one.
• Try using an older iTunes backup.

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copied from: About encrypted backups on your iPhone, iPad, or iPod touch - Apple Support