Welcome to the forums, Bernardo! I'll be happy to help, but unfortunately the Mac App Store doesn't have any support for upgrading from Standard to Professional.
The best thing to do here is going to be to contact Apple and let them know that you bought the wrong product; other customers that have accidentally purchased the wrong product have gotten a refund. Once that happens, you can purchase Professional. They've been very helpful with other customers that have had this happen.
If you contact Apple and describe what happened, they should be able to take care of this. To do so, point your browser at
this page on Apple's site, then click on "Account and Billing" in the left-hand sidebar.
If you do have any trouble getting this taken care of though Apple, or if there is anything more we can help with on our end, please email
our sales folks and we'll see what we can do. Sorry again for the confusion!