Submitted forms can be edited on the App and Portal. 

User permission must be enabled to do this, Web Portal > Users > View Users > Edit arrow for target User > User Permissions. You can then select what permissions they have for viewing and editing forms.

In the Web Portal, go to Forms > Submitted Forms > search for the Form and select the Edit arrow for it.

In the App, then go to Forms > go to the Save or Submitted tabs to search for and open an already existing form to view and/or edit.

The top right corner has a filter symbol as well, so that you can refine the search.