Welcome to the 21Winter release notes!
Before you begin the Update Checklist, there are a few things that need to be in place for your org.
First, you should be updated to 20Spring.1 or later and have your Sharing Sets configured. If you are not updated yet, please follow the directions in the 20Spring.1 Update Checklist. If you still need to configure your Sharing Sets, please follow the directions here for configuring Sharing Sets.
If your org is updated to 20Spring.1 or later and you have Sharing Sets configured, you need to complete the 21Winter Pre-Update Guide. This guide directs you how to prepare for the deprecation of the Events Master-Detail field on the Attendee object in 21Winter. If you have not yet completed the 21Winter Pre-Update Guide, it is available here.
Only when you have met these requirements should you begin the 21Winter Update Checklist to update your org. Links to the Update Checlist and other infromation on the 21Winter release are available below: