I grabbed some information for you from Google about Updating the ADT Plugin.
Updating SDK/ADT from m3-rc20/22
I'd prefer the "Alternative version", simply because it is "faster" and doesn't check all the other plugins by default.
Google wrote:Updating the ADT Plugin
To update the ADT plugin to the latest version, follow these steps:
1. Select Help > Software Updates > Find and Install....
2. Select Search for updates of the currently installed features and press Finish.
3. If any update for ADT is available, select and install.
1. Select Help > Software Updates > Manage Configuration.
2. Navigate down the tree and select Android Development Tools <version>
3. Select Scan for Updates under Available Tasks.