Yes you can upgrade from any version.
Upgradation does not mean that your current version will change to new one. If you upgrade, you can have both Elements 6 and 11 running on same computer.
Upgradation just helps you to buy the new version at a discounted rate, because you own earlier version as well.
If you want to upgrade, go to following link:
http://www.adobe.com/products/photoshop-elements.html
You may need your Elements 6 serial number. You can also buy the boxed version from nearby store.
