How do I update from CS4 to CS6?

UPgrades are never free unless you happen to buy the older version just as the newer version is being released.  Unfortunately, CS4 is not in the upgrade path for CS6.  A couple of years ago it was but then they changed it to be that only CS5 or newer would be considered.  So if you wish to have CS6 you will have to pay the full price for it.

Purchase CS6 products:

http://www.adobe.com/products/catalog/cs6._sl_id-contentfilter_sl_catalog_sl_software_sl_creativesuite6.html

You might want to consider subscribing to the Creative Cloud instead.

Creative Cloud Learn & Support

http://helpx.adobe.com/creative-cloud.html

Hi s_shell

Upgrades are available from CS5 or CS5.5 to CS6.

Details in the FAQ: FAQ | Adobe Creative Suite

I own a previous version of a Creative Suite product or suite edition. Do I get a discount if I upgrade to CS6?

Owners of a CS5 or CS5.5 suite edition qualify for upgrade pricing to a CS6 suite edition. Owners of an individual CS5 or CS5.5 product qualify for upgrade pricing on that CS6 product. Any version earlier than CS5 will not be eligible for upgrade pricing.

CS4 customers can subscribe to Creative Cloud for an introductory offer price: Cloud-based creative software and services | Adobe Creative Cloud

Thanks

Bev