Go to the Adobe store and look for Photoshop CS6. You will be taken to a page labeled something like Creative Suite.
Once there, scroll down to Photoshop CS6. Choose a platform (Mac or Win), then click on "Buy". You will then be presented with choices, one of which is Upgrade, and then you'll be shown a drop down menu where you chose the version you currently have.
If memory serves, the upgrade is about $200 ($199).