Here's how you can setup password protection for your iOS backups. 1. Connect your iOS device to your computer. 2. Select the device in iTunes. 3. Go to the Summary tab. 4. Go to Options. 5. Select "Encrypt iPhone (or iPad or iPod) Backup." 6. Type in the password you want to use. 7. Click on "Set Password."
For more, see the original article at the link below.
Password protect iPhone and iPad backups | OS X Daily