One of the most important changes is to the way shipping is handled. A lot of my clients use the Table Rates Shipping extension for WooCommerce, so I thought it would be best to explain publicly what is needed to be done next.
Once you have updated your WooCommerce plugin to version 2.6 (of course, you did a backup before you did this, right??), you will probably see a purple button in your admin asking you to Run The Updater to update your database to be compatible. Click this. It will run in the background while you do other things.
Side note – I found several sites where the Table Rates Shipping extension had not been updated, but WooCommerce had. The symptoms were that the site didn’t come up at all. There was basically a blank page. The site owner also couldn’t get into wp-admin. To fix this, I downloaded the latest Table Rates Shipping code from WooThemes, and uploaded it to wp-content/plugins manually using ftp, over-writing the live version of the plugin. This, coupled with running the database updater for WooCommerce, fixed the problem. If you are maintaining your own site, this should be within your skill set.
The main thing is to then check that you can place an order, and in particular that the shipping part works how you want it to. If not, you will need to set up your shipping again – see the WooCommerce announcement here for an overview. Or for a more detailed explanation, and a video, see this post.
Don’t forget – even if you are not using Table Rates Shipping, you could be affected. You should see this announcement in your WooCommerce -> Settings screen: