To edit a form created in the Form Designer, you don’t need to export the form. Instead, you can create a new process version and just edit the .aspx page and code-behind in Visual Studio.
First, disable the Form Designer. On the Information tab in the process information screen, uncheck Built-in form: Enable form designer.
Locate the folder where the form .aspx is stored (you can find the path in the FORM_URL process data).
The CSS in embedded in the .aspx page. You can extract it into a separate .css file if you want.
The form uses the DLLs located in the parent \bin folder, which is common for all the web forms generated by the Form Designer.
Note: This is a one-way upgrade. Changes made in Visual Studio cannot be managed by the Form Designer later.