In order to use Desktop Mode in Steam Deck, open the device, and hit the Steam button from the controller to open the menu. Now, hover over the Power option and hit A. You will see multiple options, click on Switch to Desktop. Once your device boots up, it will be in Desktop Mode. Do keep in mind that Desktop Mode is similar to what you get on a laptop as Steam Deck runs Linux. In order to exit the Desktop Mode, just click on the Return to Gaming Mode icon present on the Desktop.