You're talking about what it shows on the PS4 main menu correct?
You don't touch the fpkg. You have to edit the app.db found in System_data/priv/mms/ using ftp or ps4xplorer
You will copy the app.db to your computer and use DB Browser for SQLite (google it) once you've opened your app.db with it choose browse data > tbl_appbrowser_abunchofnumbers then double click on the game you want to rename and rename it. The only way I've found to save the changes is just x out of it, then it will ask if you want to save say yes. Then you will copy the app.db back to the same location you found it. You can just log out of the user and log back in and the changes should show.
It's a good idea to make backups of your app.db anyway. If you ever rebulild your database which sometimes can be necessary if your PS4 is acting up your FPKGs will no longer show up at all after doing so it'll be like a new PS4. The app.db will make them show back up.