Go to my.presonus in your account and copy the key to the studio one pro.
Open Studio One and stay at the start screen.
At the top of the screen, click on the Studio One menu.
Choose Studio One ACtiviation... from the drop-down menu.
Paste your product key from your my.presonus account into the box and click the Activate button beneath the product key box.
This should initialize the upgrade and take you to the studio one Installation screen.