You can only encrypt local backups. Here are some encryption methods you can use:
a)Go to Settings>Additional settings on the page with your local backups, tap the button to turn on encryption, and sign in to your Mi Account. Backups will be encrypted automatically after that. Remember that you’ll need to sign in to your Mi Account to restore items from encrypted backups.
b) If you're using Mi PC Suite to back up items you can use encryption options there.
Note: Remember the password you set for encrypted backups! You won't be able to decrypt your backups if you forget it.