Download it, install it, and use the serial number from your original purchase to activate it. You might find a download link available in your Adobe account online.
CS6 - http://helpx.adobe.com/x-productkb/policy-pricing/cs6-product-downloads.html
If you did not keep a record of your serial number, which is needed to activate the software, you might find it thru your Adobe account online if you registered it previously.