UIUC engineering undeclared is described here:
https://engineering.illinois.edu/academics/undergraduate/first-year-engineering-undeclared.html
But since you are in UIUC PREP, your path to ME or AE will be difficult, and you should make backup plans to transfer elsewhere (e.g. NIU, SIU-C, SIU-E, UIC, WIU) if you are not able to get into your desired major.