I did some reverse engineering, by following below steps now i can rename a group.
GET /groups/{name}.json
Firstly i get group info with the help of name
e.g GET /groups/staff.json
Then I have a group id which i already fetched using above method. Replaced below id to actual group id.
PUT /groups/{id}.json
e.g PUT /groups/12.json and includes a group name in your PUT request.