I was recently accepted off the waitlist for UC Davis, and decided that I would go there instead. After submitting the SIR to UCD, I was noted that I had to cancel any other SIR to a different campus and that the deadline was “immediately.” I checked on the admissions portal for UC Riverside, but could not find anything regarding the option to cancel the SIR.
See if this works:
Log in to your MyUCR account and click on the “My Profile” tab, followed by clicking on the “Cancel Application” tab and follow the instructions.