In order to restart Fire Emblem Heroes from scratch, you will need to delete your Fire Emblem Heroes data and the link to your Nintendo Account.
This data includes any Orbs you purchased/obtained, Heroes, and other items. Your data cannot be recovered after it is deleted.
Follow these instructions to delete your Fire Emblem Heroes data.
❶ From Menu on the title screen or the Misc. menu within the game, select Account Management.
❷ Select Delete All Data.
After deleting all your data, delete the application.
To restart Fire Emblem Heroes, reinstall the app to begin from the start.