Note:
To know your instance, check https://www.infallibletechie.com/2016/10/how-to-know-salesforce-server-instance.html
2. Check whether any Remote Site Settings has hard coded instance name.
3. Check whether instance name is hard coded in custom label, custom settings, custom metadata type, etc.
4. If you will need to view your Email Logs after the instance refresh and are moving to a new data center during the instance refresh, then you need to request your Email Logs prior to the maintenance window.
5. The sandbox refresh queue will be suspended 4 hours before the instance refresh maintenance begins. If your org was in the midst of a sandbox refresh when the queue is suspended, the sandbox refresh will be stopped. Your sandbox refresh will be restarted (not resumed) 24 hours after the instance refresh maintenance has completed.
6. How to avoid hard coding of instance name like NA12 for the documents images in Salesforce?
https://www.infallibletechie.com/2018/09/how-to-avoid-hard-coding-of-instance.html
7. If Email Relay is used in your org, make sure the IPs are whitelisted in the Email server.
8. Smoke test the applications post refresh.
Knowledge article from Salesforce
https://help.salesforce.com/articleView?id=Instance-Refresh-Maintenance-FAQ&language=en_US&type=1