I think you need to email infoservice@magix.net and ask them.
However, when you say upgrade your subscription do you mean you want to renew it? I ask that because it would seem that Magix/Xara has decided that there will be no further updates or upgrades to WDP; as far as I am aware the upgrade route to any "new version" requires the user to cross-grade to Xara Designer Pro. This page shows Designer Pro as the only "update" available for Web Designer and Photo & Graphic Designer!
You now need to buy the subscription directly from Xara. Magix customers can get a 40% discount for the first year as far as I know. Magix no longer sells Web Designer.