There is no automated upgrade process due to the nature of Umbraco. Once a theme has been installed we cannot account for any changes you may have made. There would be a danger that any changes you have made would be overwritten.
Each theme has its own specific updates log and this will indicate to you what has changed between versions. Using this, you will be able to identify what has changed and then you can copy the files from the latest package version to your own website, manually. For more complicated changes between versions, the best approach would be to use a program like WinMerge (http://winmerge.org/?lang=en). Setup a new Umbraco website using the latest uSkinned Package and compare this against your current website.
It is important to understand that our themes are Starter Kits. They are the starting point for your own website providing you with a huge amount of functionality that allows you to get a website up and running quickly. We will always release new updates and features to each of our themes so the best way to look at our upgrades is that these contain more new features for the launch of your next website.