In PADS Layout go to Setup > Pad Stacks... and click on the Via in the Pad Stack Type.
In that dialog you can add/edit/delete your via...
Based on your feedback, I'm going to add information to the PADS Router documentation to explain that editing or adding a via can only be performed in PADS Layout. I'll add links to the procedures in the PADS Layout user's guide.
Thanks for reporting on this difficulty.