Encrypt your iPhone backups
Turn on encryption for iPhone backups in iTunes
Here's how you can encrypt your iPhone backups in iTunes.
1. Connect your iPhone to your computer.
2. Select your iPhone in the sidebar.
3. Go to Summary tab.
4. Go to Options.
5. Click the checkbox next to "Encrypt iPhone Backup."
For more, see the original article at the link below.
Encrypt your iPhone Backups | OS X Daily