How to Edit saved search using Splunk REST ?

I want to edit the search of a Saved Search Report using REST in Python without any other change. But when i am using the below code,it is creating a new saved search with private permission. Please help me in changing the search of the saved report with the existing permission. myhttp.request(baseurl + '/servicesNS/sys-splunk/AS/saved/searches/15_report','POST', headers={'Authorization': 'Splunk %s' % sessionkey},body=urllib.urlencode({'search': "index=*"}))