You’re not the first person who has made a change, only to have regrets later.
There are many changes in life that can be undone. The first thing you need is information. Call UIUC admissions and find out whether it is possible for you to come back, and when. Then talk to DePaul to see if you can get out of whatever obligation you have with them. If you end up staying for the semester, get the most out of it that you can.
People change jobs, careers, neighborhoods, whatever. IMO life is too short to spend three years somewhere that you don’t want to be. Good luck!