Don't bother with the VM web site, it is pants. Phone VM up and select the options for "I'd like to change my package" or similar. They're usually very helpful, although be aware you'll start a new 12 month contract lock in, and you will only be offered the currently tiered 50/100/200/350 Mbps speeds, so if you're on a 60, 70 or 150 Mbps legacy contract, that speed option will disappear forever.
Also, they don't appear to offer 350 broadband only upgrades online even when their web pages work - they try and push you to take a TV package as well, so that's another good reason to speak to the sales staff. Incidentally, looking at the problems some people have had with 200 to 350 upgrades, I wouldn't touch the 350 offer, but that's just my view.