If you need to rotate your OAuth client secret, you can do this by logging into iRacing Account Management at https://oauth.iracing.com/accountmanagement/ and selecting OAuth Clients on the left-hand menu. If you have 2FA enabled and had previously selected Remember Me, you will be prompted to log in again before the accessing this page.
After clicking the Manage secrets button, you will be able to generate new secrets (up to 5) and revoke any existing secrets.