def refresh():
    """
    Refreshes an existing JWT by creating a new one that is a copy of the old
    except that it has a refrehsed access expiration.

    .. example::
       $ curl http://localhost:5000/refresh -X GET \
         -H "Authorization: Bearer <your_token>"
    """
    old_token = guard.read_token_from_header()
    new_token = guard.refresh_jwt_token(old_token)
    ret = {'access_token': new_token}
    return flask.jsonify(ret), 200