Please see https://releases.xamarin.com/stable-release-cycle-9-with-xcode-8-3-support for the details about this release, including the steps to update or downgrade as needed.
For Visual Studio 2017 users, be sure to complete the manual steps to download and update to this version.
Please file a quick bug report using the "Cycle 9" 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: