First, verify that you have enough credits to activate a device as premium. You need 365 credits for each device.
Next, check the box next to the device you want to activate as a premium device.
Click Actions to the right and select Upgrade to EcoStruxure Asset Advisor ... to activate it as a premium device.
You will see a crown next to your device to indicate that it is now a premium device and is subscribed to EcoStruxure Asset Advisor.