switch to the unstable repository and use apt-get update && apt-get upgrade and you'll get the latest build, assuming there is one for your platform/architecture. Be aware that (a) they are unstable, so bugs are to be expected (pre-alpha, under active development, although it's pretty reliable for the most part) and also (b) you'll get an update with pretty much every new code push, which will mean updating regularly.
If the latter scares you, build it yourself by following the instructions in the wiki. If the former scares you, use Kodi instead of Plex and stay on 3.4...
