November 3rd, 2014: For information on what's new in 0.9.1, see the release announcement.
If you are already running Android Studio version 0.8.12 or later, manually check for updates via Help > Check for Update... (on OSX, look in the Android Studio menu). This will download and install a small patch rather than download a full IDE image. For all older other versions you will need to install it from scratch by installing one of the below .zip files.
To install Android Studio manually:
Windows: https://dl.google.com/dl/android/studio/ide-zips/0.9.1/android-studio-ide-135.1561280-windows.zip (170 MB)
Mac: https://dl.google.com/dl/android/studio/ide-zips/0.9.1/android-studio-ide-135.1561280-mac.zip (170 MB)
Linux: https://dl.google.com/dl/android/studio/ide-zips/0.9.1/android-studio-ide-135.1561280-linux.zip (169 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. We no longer recommend keeping the SDK installation inside the application folder, so after moving it out of the way, please place it somewhere else (such as your home directory), and then point to it from the IDE's project structure dialog.