You'll have to install the original version of Apsona that you have, first.
This page on their website explains how to install the update for 1.5 on top of the original Apsona install, along with a link to download the update: Apsona 1.5 update.
Also, with the update to 1.5 there isn't a link in the admin section of your store to access Apsona anymore. You'll have to create a bookmark to the page & save it to access it.
Your bookmark will be the following: http:// YOURWEBSITE.com /YOUR ADMIN FOLDER/apsona_index.php#home/dashboard
(If you're not already logged in to your admin area, the sign-in page will pop up & you'll have to log in to access Apsona.)
Hope this helps you out :)
Bookmarks