Close Animal Crossing: New Horizons
On the Nintendo Switch home screen, highlight the game
Press the plus button
Select Close
Reopen the game
To fully restart your island, back up any save data you want to keep first
From the Nintendo Switch home screen, go to System Settings
Select Data Management
Select Delete Save Data
Choose Animal Crossing: New Horizons
Confirm the deletion
Launch the game again to start a new island
To restart from the title screen, press the minus button and follow the reset options if available