You can add contacts on your account at inreach.garmin.com and sync them to your device. You can also add contacts on the device, but those contacts are not synced with your online account and therefore not backed up.
This article explains how to delete contacts on your inReach.
1. Select Contacts and press
2. Select a contact from the list and press
3. Select Delete and press
4. Select Delete and press to delete the selected contact.