Upgrading from m3-rc20/22 requires the following steps:

1. Download and unpack the latest SDK (:src: here).
2. Update the ADT plugin according to the instructions below.

Updating the ADT Plugin

To update to the new plugin you will need to:

1. In Eclipse, select Help > Software Updates > Manage Configuration.
2. Find the ADT (Android Development Tools X.X.X) plugin on the tree in the left panel. Right click and select Uninstall. Agree to restart Eclipse.
   
   If you mistakenly chose Disable instead of Uninstall, the plugin will not be deleted but simply disappear from the list since by default, disabled plugins are not shown. You can show them again by choosing Show Disabled Features in the toolbar (3rd icon from the left). If you want to fully delete the old plugin, you will have to re-enable it and choose Uninstall again.
3. Eclipse may tell you that it cannot restore the workbench layout. You can ignore those messages. Additionally, if you have the DDMS perspective icon in the upper right corner, you should right click on it and select Close to remove it.
4. Follow the standard instructions to install the new version. Due to this particular install process, you will need to enter the location of the SDK in the Android preference page again. (See step 11 in Installing the Eclipse Plugin.)

:!:
Note that your Android projects will not be recognized by the new plugin. To fix this, right click your project in the Package Explorer and choose Android Tools > Convert To Android Project.