Using the Marathon REST API to update a configuration

Using cURL I want to modify the number of instances a group has by passing a json file to update the config. The way my groups are set up looks something like this:

'marathon > applications > topApp'
'marathon > applications > topApp > group1'
'marathon > applications > topApp > group2'
'marathon > applications > topApp > group3 , etc'

I only want to update group2 and cannot figure out how to specify it. I'm thinking of using 'PUT /v2/groups/{groupId}' which I got from the marathon rest api doc here https://mesosphere.github.io/marathon/docs/rest-api.html#example-5

Has anyone done this and can help? Is there a better way? This is my first time doing this. Thanks!

Copyright Notice：Content Author:「donniemac」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/43517695/using-the-marathon-rest-api-to-update-a-configuration