It should be the same way you've changed it, click on your username icon and press on mobile version, if the option isn't showing, you might be a bit cooked. It's one of the website's glitches, you might need to log out(close all tabs) and give it a second to refresh. Or, you can log into the website on another device and see if it's on the mobile version there, if not, change it and go back to your og device and refresh. BUT! Don't rely much on my advice, I'm just suggesting this method. I was stuck on the desktop version for awhile, and still don't know what really helped.

Does anyone knoq how to undo the desktop version in my phone?