It's not possible to convert an organization into a personal user account, but you can create a new user account and transfer the organization's repositories to it.

  1. Sign up for a new GitHub Enterprise user account.

  2. Have the user's role changed to an owner.

  3. Sign in (https://[hostname]/login) to the new user account.

  4. Transfer each organization repository to the new user account.
  5. Delete the organization.
  6. Rename the user to the organization's name.