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 user account.
  2. Have the user's role changed to an owner.
  3. Sign in 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.