Just tried it here with Kicad 5.01 under Linux and I could edit the pad and hole.
Select the via then select "properties" (or press 'E' on the keyboard)
You should get a panel showing "track and via properties".
In it there are settings for "Diameter" and "Drill".
I tried 2.5mm for the pad diameter and 0.6mm for the hole, it seemed to work fine and only on the selected via, the others stayed as they were.
Make sure the via type is set to "through" and not "micro" or "blind".
Brian.