Upgrading is as simple as downloading the new version (either the "update" or the "full version"), and installing it over the old one. That will update any installed plugins, too.
One important thing to keep in mind when you upgrade from the version you have:
If any of your dimmable modules (lamps and/or wall switches) are the older ones (without the SoftStart feature - see below for an explanation), you will need to edit the modules, and change the category from "Lamps" to "Old Lamps - NoSoftStart," and then re-select the module type. Mixing both "old" and "new" modules on the same HouseCode/UnitCode combination (ex: two lamps in the same room you want to go on/off together) is not recommended. The modules will not work correctly if you do. In that case, you should either switch the mis-matched modules around so they are all the same type, or define them all as "Old Lamps."
If you aren't sure if your modules have SoftStart or not, see my explanation below (you can skip the rest of this post if you already know what SoftStart is, and how to tell if your modules have it).
Explanation of SoftStart, and the ramifications of picking the wrong module category:
Back around 2008, X10 changed the design of their dimming modules (plugin lamp modules and wall switches), and added in the "SoftStart" function and the ability to be controlled using "Extended Dim" commands. The new design allows you to set a module to a specific dim level directly, without having to go to "full on" first. The new modules will slowly "ramp up" to the desired level when turning on, instead of going on immediately at full brightness (like the older ones). The newer modules will respond to "Extended Dim" commands, but the old ones won't. However, the new modules will still respond to the older "Percent Dim" commands just fine. Unfortunately, they didn't change the model numbers when they changed the design. The easiest way to determine which ones you have (and you might have a mixture of both) is to watch them when they turn on. If they immediately jump to full brightness, they are the older ones. If they "ramp up" to full brightness, they are the newer ones.
Starting with version 3.302, X10 added support for the newer (SoftStart) modules. Since the module numbers are the same between the old and the new, they decided the "Lamps" category should be the newer ones (the ones any current customers would be buying), and added a second category, "Old Lamps - no SoftStart," for the older ones. AHP sends Extended Dim commands to control the newer modules, and it sends Percent Dim commands to control the older ones (the "off" command is still the same for both).