<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def update_shortlink(self, shortlink_id, callback_uri=None, description=None):
 """Update existing shortlink registration Arguments: shortlink_id: Shortlink id assigned by mCASH """

        arguments = {'callback_uri': callback_uri,
                     'description': description}
        return self.do_req('PUT',
                           self.merchant_api_base_url + '/shortlink/' +
                           shortlink_id + '/', arguments)