Once you have created a new instance for a WebForm, you cannot normally change the template behind the form. You can, however, clone the form to a new version, which will be based on the latest version of the original template.
To do this, you will first need to open the WebForm Designer for the template and select Actions > Clone and Retire.
You can then make the necessary corrections to the template before Saving as usual.
Once you have created a new version of the template, you can clone the WebForm instance to the new version. To do this, find your form in the WebForm List and select ... > Clone to New.
This will open a window displaying the Title and Owner of the form and explaining the cloning process. Select Clone to New to confirm or Cancel to go back.
After cloning, the original form instance will have its status changed to Deleted and you will be taken to the WebForm Summary for the new form instance.
For more information, take a look at the full help page for Cloning WebForms.
Keywords: Fix WebForm template, error in form file, change WebForm design