You can retake anywhere, but they have to be same or greater course number and same or greater semester hours.
I'd check local community colleges for potential replacements before staying at your UG to retake. but like @hallowmann said, it's easier if it's at the same institution because you KNOW it will count. that and you get to spend some more time at a school you hopefully love and can continue relationships with your letter writers, etc.
how many classes do you need to retake? maybe you are close to a minor that you could get while you're retaking your other courses?
i'd suggest taking as close to 12 hours as possible though so that you have the most amount of time to work on app, etc.