I mistakenly put down selected RD instead of REA. What should I do? Will they consider my application RD instead of REA now?
You still have time to change it. This is on their website:
Change of Application Plan
If you originally applied under Regular Decision and would like to be considered as a Restrictive Early Action applicant, follow these instructions:
Request a change in status by November 1 via email to credentials@stanford.edu. Applicants must confirm the request has been received to ensure the change has been made.
Withdraw an early application you may have submitted to another college/university.