If you need to change the rejection reason on a group of candidates that were rejected in bulk, you will first need to un-reject the candidates and reject them again with the correct reason.
You can't un-reject candidates in bulk in Greenhouse Recruiting. However, if you use the Harvest API, you can un-reject a cohort of candidates using
Unreject application, and bulk reject them again with the updated reason.
Click here for more information on the Harvest API.