Please see https://releases.xamarin.com/beta-release-15-1-beta-preview-1/ for the details about this release, including the steps to update or downgrade as needed.
This release uses a new naming convention "15.1" that more closely resembles the sub-version numbering of Visual Studio 2017. "15.1" in this new scheme matches up with what Xamarin would have called "Cycle 10" under the old scheme.
Note that this preview release is not yet available in Visual Studio 2017. It will be integrated into the Visual Studio Installer at a future date that follows the upstream VS 2017 release schedule. Xamarin coordination with the upstream schedule has been evolving across the VS 2017 releases, and plans are underway to synchronize more tightly as soon as possible. Watch for additional news on that topic over the next week.
Please file a quick bug report using the current "Feature 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: