Please see https://releases.xamarin.com/beta-release-15-3-preview-4/ for the details about this release, including the steps to update or downgrade as needed.
Note that this Beta version is a smaller release compared to the current Alpha preview. In particular, it does not include the Xamarin Live Player, Docker, Azure Functions, or Xamarin IoT preview features announced at Build. Please stay on the Alpha channel if you wish to continue to preview those features.
Update for June 29: The Xamarin extension versions included in the latest Visual Studio 2017 Preview version 15.3.0 Preview 3 are for the moment still the earlier Beta versions from June 26. The new matching Xamarin versions will be integrated into the Visual Studio Installer at a future date that follows the upstream VS 2017 Preview release schedule.
Please file a quick bug report using the "15.3 Preview" new bug form if you see any suspicious behavior in these versions that you wish to report.
Comment in this forum thread if your question is not related to a suspicious behavior after download, but instead one of the following:
EDIT June 26: Update for Visual Studio 2017 Preview version 15.3.0 Preview 3 availability.
EDIT June 29: Update link to Release Blog post for new versions.
EDIT July 12: Update link to Release Blog post for new versions.