There are two ways to do this: by going through a Judge Profile, or by going through a Competitor Profile.
First Way: Modify Judge Profile
Navigate to the Judges tab. Find the Judge in the list and click on their name. In the popup, add or remove Competitors in the "Assigned Competitors" box. Click Save to save your changes.
Second Way: Modify Competitor Profile
Navigate to the Competitors tab. Find the Competitor in the list and click on their name. In the popup, add or remove Judges in the "Assigned Judges" box. Click Save to save your changes.
You can make changes before, during, and after a competition, as well as before or after the RocketJudge auto-assignments are made (learn more about that here).