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Get the installation package from the download site at http://vaadin.com/download. If you are using the Vaadin Plugin for Eclipse, upgrade the Vaadin version from the project preferences. If using Maven, the repositories will replicate in a few hours. The offline Vaadin Plug-in for Eclipse has also been updated to the new 6.6.8 and will shortly be available for download.
As always, when upgrading from an earlier version, you should recompile any custom widget sets and refresh your project in Eclipse. See the General Upgrade Notes for more details on upgrading.