Log out and then log back in, it should return to mobile version by default. If not, select any manga, and click on start reading. There should be a "New Version" on the top left of the page. If it still doesn't work, check your browser because you might have enabled "Desktop Site" by accident.
Yoooo, I accidentally went back to the mangago old version and now I can't find the switch to new version anywhere, what do I doooo