Using New Outlook on Windows for personal email, calendar, and contact management
For New Outlook this feature is currently released to Targeted Release at 100%. It should start going to production any day now. I had previously put my test tenant users on Targeted Release, and I do see the feature now when updating meetings. If you urgently need this functionality, ask your Tenant Admin to put you on Targeted Release, Set up the Standard or Targeted release options - Microsoft 365 admin | Microsoft Learn. It's not complicated to for the Tenant Admin to do this so should be a reasonable request.
Here is the dialog I saw today when testing it. Looks like the button is not drawing properly on my machine but it worked fine.