Submitted forms can be edited on the App and Portal.
User permissions must be enabled to do this, your Formitize account Administrator can check this in the Web Portal by going to Admin > All Users > Edit arrow for target User > User Permissions. You can then select what permissions they have for viewing and editing forms.
To edit a Submitted form in the Web Portal, go to Forms > Submitted Forms > search for the Form and select the Edit arrow for it.
In the App, log in and then go to Forms > select the Saved 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.