How to delete a game file?

Note for future players: deleting your save file will wipe all Pokemon, including any Pokemon you may have gotten from events, Mystery Gifts or other one-time offers. Be absolutely sure you want to do this before proceeding!

You can reset your save game from within the Nintendo Switch Settings (i.e. not in-game):

• Open System Settings (from the Nintendo Switch Home Screen)
• Find 'Data Management'
• Scroll down to 'Delete Save Data'
• Choose Pokemon Sword & Shield to delete your save file
• Select the Switch profile you wish to reset

Sources:

• GamersHeroes.com - How to restart a new game in Pokemon Sword/Shield
• AttackOfTheFanboy.com - Pokemon Sword and Shield – How to Start a New Game

In the settings, go to Data Management, and then Delete Save Data. In there, select Pokémon Sword or Shield, and select the user. In this way, only the save game for one user will be deleted.