Please see https://releases.xamarin.com/stable-release-15-1/ 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.
Please file a quick bug report using the "15.1 Release" new bug form if you see any suspicious behavior in these versions that you wish to report.
You can also check the list of issues reported by users so far for the 15.1 Stable channel versions to see if any match. If you find a match, feel free to CC yourself on the bug report as a way to tally your interest in the bug:
(If you have reported an issue that is specific to the latest 15.1 release but is not yet included in the list, the bug might not have been filed using the "15.1 Release" new bug form. Those bugs will be added to the 15.1 list as they are reviewed by the team.)
Comment in this forum thread if your question is not related to a suspicious behavior after download, but instead one of the following: