At the login screen, click on the user first and then click the gear symbol and select Xfce session to login to use Xfce desktop. You can use the same way to switch back to the default Ubuntu desktop environment by selecting Ubuntu Default. At the first run, it will ask you to set config.

What is the default Ubuntu desktop environment?

From Ubuntu 17.10, GNOME Shell is the default desktop environment. From Ubuntu 11.04 to Ubuntu 17.04, the Unity desktop interface was the default. A number of other variants are distinguished simply by each featuring a different desktop environment.

How do I change my default desktop?

Find your “Desktop Personalization Settings.” Turn on your computer and wait for your desktop to load. Right click on your desktop and click on “Personalize” to be taken to your desktop settings. Click “Change Desktop Icons” under “Tasks” and double click “Restore Default.”