If you wish to edit a web forward that you have already entered, you need to login to PowerDNS first and click on 'Edit' link next to the domain you want containing the record you want to edit.
You will then be taken to the 'Domain Control Page'.
On this page you should click on the link 'Web' just above the 'Domain Details' section. You will be redirected to a Domain Control Panel section dedicated for your web forwards.
If you want to edit your webforward 'stuff.my-domain.com' , click on 'Edit' next to it.
You will then be taken to the 'edit a web forward form. Here you can change your web forwards information. When done editing the web forward you will be taken back to the 'Domain Control Panel' page.