The latest version of the Atlassian Plugin SDK is now available on the Atlassian Marketplace . So please feel free to update at your convenience.
This version includes AMPS 6.3.21.
You can find the release notes in Atlassian Marketplace.
APT-Get users please remember that after our recent changes to the way sdkrepo is hosted you will no longer be able to update using the apt-get tool. You should uninstall your Atlassian Plugin SDK using apt-get and then re-install the latest version using the instructions found in the documentation: Install the Atlassian SDK on a Linux or Mac system