Since CMS forms use the stand-alone forms found under Communities > Forms, updating this stand-alone form will update its corresponding form in CMS. 

If you are looking to edit the template or wrapper content, then those edits will need to be done through the CMS management tool.  For each page and piece of content you can choose to have a unique or shared custom template and wrapper.  To select these options for a CMS page:

1.   Go to: Content > Content Management System
2.   Hover over the event name and click: Manage Website
3.   Click Content Management tab: Site Structure > Folder
4.   Expand the section that contains the form that needs to be edited
5.   Click on the desired form
6.   Update the Title on the Properties tab
7.   Go to the Advanced tab
8.   Expand the Design Option area and you will see that you can choose from pre-existing templates and wrappers

NOTE: You can also upload them directly through this same area. Once a file is uploaded here, you can edit any other page and that option will now be part of the drop-down selection menu.

To edit an existing wrapper
1.  Click Edit under Wrapper Content
2.  Click the Body tab
3.  Add/Update the text, images etc.
4.  Mark the checkbox labeled Publish Immediately
5.  Click Update, the window will close
6.  Click Apply then OK
7.  Refresh the webpage.

To edit the form associated with this web form:

  1. Find and click on the web form in site structure
  2. Click Event Selection
  3. Choose the correct event
  4. Choose the option for Publish Immediately and click OK