I suppose you are using Samplitude 11.5 Producer?

Anyway, there is a link in the help menu called "Upgrade to Samplitude". As you can see above (click on the picture to see the full screen).

Click on it and clik "Upgrade now!" in the window that will pop up.

You will be forwarded to a page with a little video and your advantages and can go to the shop from there.

Have fun.