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 update a rejection reason using PATCH: Update Rejection Reason endpoint.
Click here for more information on the Harvest API.
Tip: If you are rejecting a candidate a second time, be mindful of any automated notification emails that may be sent to candidates. You can choose to not notify candidates of their rejection when sending the email.