The credentials for you to log in to the WebDAV direcotry, which is where your backup is located, are the same credentials (username and password) you use to log in to JIRA and/or Confluence, however, you must have JIRA Administrators global permission (or Confluence Administrator global permission if your instance is Confluence-only) for the WebDAV directory to accept your log in.
If you don't have it, ask one of your instance's current admins to download the backup for you or to grant you admin permissions so you can download it yourself.
If you do have global admin permission but you're still unable to log in, then open a support request at https://support.atlassian.com for us to investigate it.
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