At this point the upgrade workflow is not functional just yet, as mentioned here: http://forums.adobe.com/docs/DOC-2153
So the webBasics Creative Cloud cannot be upgraded directly to the webBasics+ level for now.
But a workaround for this case would be to re-publish your Dreamweaver/Muse project (I suppose you've used one of these solutions to build the site in the first place, right?) in a new trial/temporary site -> let us know what is the URL of that site (don't push it live) -> we will change the site plan from our backend to webBasics+ -> you will be able to publish it using the webBasics+ site plan.
Please note that this is going to be a separate charge, besides your Creative Cloud subscription. The monthly fee for webBasics+ will be $12.21/month, as mentioned here: http://www.adobe.com/products/business-catalyst/buying-guide-subscript ions.html
Another way to use the webBasics+ plan would be to simply trigger a new trial site here: https://syd.worldsecuresystems.com/PartnerPortal/FreeTrialSignup.aspx# splash with the same Adobe ID as the one used to buy the Creative Cloud subscription and take that site live separately.
