You might need to turn on early adopter but you should be able to do the upgrade by logging into plesk, go to Tools and Settings, clicking on Updates and Upgrades, and click on Install or Upgrade Product. From there you should be able to upgrade to 12.5. If the option is gray out then the upgrade option is not yet available for stable release (even though it's officially out now) and will need to enable adapters which can be do by going back to the Tools and Settings and clicking on Update and Upgrade Settings.