Is it possible to restart Fire Emblem Heroes from scratch after linking a Nintendo Account?

You can restart from the beginning by deleting all your Fire Emblem Heroes save data. 
Please follow these steps after carefully reading the notes:

■ Notes

• All Orbs, Heroes, and other items you have will be lost.
• You cannot retrieve your save data once you delete it.

■ Steps

1. Tap Menu on the title screen or Misc. in the game, then tap Account Management.
2. Carry out the Delete All Data process.
3. Uninstall the application.
4. Reinstall it from the store.