You can add contacts on your account at explore.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 edit contacts on your inReach.
1. Select Contacts and press
2. Select a contact from the list and press
3. Select any of the contact fields and enter the updated details.
4. Select Done and press when finished.