If you are not using SSO and need manually reset or send a judge a new password, you can do so by clicking on Judges in the left side navigation and then click Search.
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If you are using SSO your judges will need to reset their password using your website.

Search for the judge on the following screen.

Click Select on the judge you want to impersonate.

Tab over to Tools and then choose between Auto-Generate and Email Password or Manually Set and Email Password. The former will create a random password and email it to the judge while the latter will let you specify what the password is.