1. Backup your database first.
2. Download version 8.x from https://wordpress.org/plugins/front-end-pm/advanced/ and upload in your website.
3. Visit your website admin area. It should redirect you to update page. If not visit Front End PM > Settings > Update. Wait until finish.
4. Visit your message page and see if you see your messages. It YES, delete database table starting with PREFIX_fep_. If you do not see your messages then do not delete database table. let me know instead.
5. Now update to latest version. After update you should see an admin notice for database update. Click “Proceed”.

Let me know.

My next answer may delay as i may go outside of internet. Expected available date 5th march