At one point I tried the pantheon desktop environment (I don't remember doing anything weird) and then suddenly I was not running Ubuntu anymore but elementary. I never used a bootable usb to switch the OS or anything like that, can I go back?
SnapOverflow
I don't know pantheon but if it is a DE as you say, and you just installed it, as you also say, it should just be a matter of selecting the DE you prefer from the "display manager", aka the graphical login screen, assuming you have one. There should be a drop down menu on it somewhere allowing you to select DE. Installing a DE doesn't uninstall other DEs. Not normally at least. If you don't use a display manager, like light-dm or whatever, but start you gui with startx from the command line, try looking at the man page for the DE you like. It will probably tell you what to append to startx to boot into it. If you start the gui one of the other ways, I can't help you.