It is listed in the docs here: https://parse.com/docs/rest#objects-updating
curl -X PUT \
-H "X-Parse-Application-Id: app-id-here" \
-H "X-Parse-REST-API-Key: rest-api-key-here" \
-H "Content-Type: application/json" \
-d '{"score":73453}' \
https://api.parse.com/1/classes/class-name-here/object-id-here
where the data is a json encoded object of the updated keys.