Managing save data/screenshots
Open Album on the HOME Menu, then press the X Button on the list of screenshots to select images to delete.
If you'd like to manage all of your screenshots at once, go to System Settings ⇒ Data Management ⇒ Save Data/Screenshots ⇒ Manage Screenshots. There you can delete all of your screenshots or copy data between the microSD card and the system memory.
All of your save data will be saved in the system memory.
You can delete save data for each individual software via System Settings ⇒ Data Management ⇒ Save Data/Screenshots ⇒ Delete Save Data.
Deleting all data
You can delete all data and return the Nintendo Switch console to the state you bought it in by restoring factory settings.
All information saved on the Nintendo Switch console, including user information, internet settings and game-related news articles, will be deleted.
→Restoring the console to factory settings