Microsoft will update your environment to the new major version in its window; you pick the day.
Admin centre → the environment → Update settings: the update window (the hours of the day updates may run) and, when a major version is available, Schedule update with a date within the allowed period (usually about five months from availability). Pick a quiet day, not month-end, not peak season, after you have tested in a sandbox on the new version. Minor updates arrive monthly within your window without a choice. Set the notification recipients on the admin centre so the right people get the "update available" and "update failed" emails, not the person who set up the tenant three years ago.
Miss the deadline and Microsoft updates you anyway, on their day. The release wave checklist is the eight weeks before this click.
