I applied to 2 universities and got into both. I accepted one of them but after accepting their offer, the other university gave me a good financial aid offer. Is there any way I could "un-accept" the university I already accepted and accept the other?
I think you’d just contact the admissions office and tell them you’re no longer going. You might lose your deposit (if you paid one), but that’s about it.
Before May 1st, I think you still have the option to pay a different deposit and let the first college know (though you may lose the original deposit). Double-check with your college what their policy is. However, do try to get in contact with your current college's office of financial aid. Tell them you've had a better offer that unless they can match, you will accept and transfer to that school instead. Oftentimes they are willing to put in quite a bit of effort to keep you if you're a competitive applicant who has shown interest.
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Yes most of the deposits are non refundable from what I have seen