How to use Desktop Mode in the Steam Deck

Step 1: Turn your Steam Deck on, then once it’s booted up, press and hold down the Power button. Hold it down until a new menu appears

Step 2: Select Switch to desktop.

Step 3: Technically, your Steam Deck is now in desktop mode. However, using a keyboard and mouse will make working in the mode much easier (the trackpad is OK but not amazing). A laptop or monitor connection is also advised so you can use a bigger screen for improved vision.

Source: https://www.digitaltrends.com/