Just download Prime7 and install. Don't (try to) enter a license. That's it, you have Prime 7 express.
If you want to make use of the trial period to experience the full feature set, you do need to get a license. This will expire after 30 days and from then on it functions as Prime 7 express.
Note that you do not need to uninstall Prime 6. You can run all prime versions side by side (The only exception is Prime 3 and 3.1), but you have to install them in the order of releases.
For more information on dowloading and installation see here.