GUI installer (requires Swing):
i2pinstall.jar (SHA1 34962d346bf607c99d7b67e484dcfaea2ea28711 sig)
Right-click "save as", then either double click on it or run java -jar i2pinstall.jar
Headless install:
i2p.tar.bz2 (SHA1 faabd06d081e1ff338b83bf2c52077cb1a6f8c26 sig)
Run (tar xjvf i2p.tar.bz2 ; cd i2p ; vi install-headless.txt)
Source install:
i2p-0.5.0.5.tar.bz2 (SHA1 9e7cb30bdf7b061ed10193fe8b031bb04503c7ee sig)
Alternately, you can fetch the source from cvs (CVS HEAD gets you the most recent updates, while the tag i2p_0_5_0_5 gets you the release)
Run (tar xjvf i2p-0.5.0.5.tar.bz2 ; cd i2p-0.5.0.5 ; ant dist) then either run the GUI installer or headless install as above
After running the GUI install on windows, simply click on the "Run I2P" button which will bring up the router console, which has further instructions. When first installing, please remember to adjust your NAT/firewall and reseed.

With the new announcement system, you should see a link on your router console allowing you to download and install the new release by just clicking on that link. There is a bug in the 0.5.0.4 code though, so you may need to delete the file docs/news.xml from your I2P installation directory and wait a few minutes for a new news.xml to be retrieved.
Alternately, you can use the instructions specified below.

Download i2pupdate.zip (SHA1 549fb362c7bceee5c4bf9063c7280a650242851f sig) to your I2P installation directory. (alternately, you can get the source as above and run "ant updater", then copy the resulting i2pupdate.zip to your I2P installation directory)
Click "Graceful restart"
Grab a cup of coffee and come back in 11 minutes

Sorry, no upgrade available. Please do a full clean install.

If you're having trouble, swing by the forum, post to the mailing list, or get on irc.freenode.net/#i2p / invisiblechat/#i2p.