April 16, 2014: For information on what's new in 0.5.5, see the release announcement.
You can install 0.5.5 incrementally by just using "Check for Updates" in your IDE, and applying the update mechanism which will patch your existing install.
To install Android Studio manually:
Windows: http://dl.google.com/dl/android/studio/ide-zips/0.5.5/android-studio-ide-135.1122749-windows.zip (159 MB)
Mac: http://dl.google.com/dl/android/studio/ide-zips/0.5.5/android-studio-ide-135.1122749-mac.zip (158 MB)
NOTE: These .zip files do not contain an embedded SDK install. If you have an existing install of Studio which contains an embedded SDK, if you upgrade by installing one of these zips, make sure you copy over the SDK as well.