If you are upgrading from Studio One 3 then you will need to download the Studio One 4 installer from your PreSonus account.
Once downloaded an installed please do the following :
Please do NOT enter a product key or check the Upgrade box. Run Studio One 4 and follow the steps below...
1. Go to menu Studio One > Studio One Activation (if it launches directly into the activation screen go to step 2)
*** If you've never run Studio One on this system before it may launch directly into a log in screen or it may pop up later. If so, be sure to log in with the credentials for your user account - firstname.lastname@example.org + this account password or activation will not work. ***
2. Check the option ... Activate my purchased version of Studio One. Do not check the box "I have purchased an upgrade of Studio One”
3. Click the downward facing arrow on the right edge of the list field there to refresh your product keys listing with Show My Existing Product Keys. This will retrieve the registered products from your user account.
4. Select Studio One 4 Professional from the refreshed list and click Activate and it will activate the new license and restart as Studio One 4 Professional.
Hope that helps.