Once forms go live it is technically impossible to edit them. However, forms can be duplicated and those duplicates can be updated and changed. This guide will show you how this is done.
Firstly, you need to create a duplicate of the form you wish to edit. In the Content Manager, go to Forms, and then duplicate your form.
This will create a copy in draft. Click view:
You will then be able to make your changes to the form, including to the heading.
Once you've made your changes, click Make Live.
Go back to the older form and delete it.
Note: If the old form was in a pack or workflows, you will need to update those manually and add the new form back to those.
Hope this helps!