Here's how you can enable encryption for your Time Machine backups in OS X.
For new Time Machine backups:
1. Connect the backup drive to your Mac.
2. Click the checkbox for "Encrypt Backups."
For existing Time Machine backups:
1. Go to System Preferences.
2. Go to Time Machine.
3. Go to Options.
4. Select your Time Machine backup drive.
5. Select "Encrypt Backup Disk" or "Encrypt Backups."
For more, see the original article at the link below.
Encrypt Time Machine Backups with OS X | OS X Daily