To view Calendar access rights:
Get-MailboxFolderPermission -identity "ManagingUser:\Calendar"
You must remove any user listed as Reviewer before you can add Editor rights.
Remove-MailboxFolderPermission -identity “ManagingUser:\Calendar” -user “PersonalAssitant”
Once Removed Add Editor Access rights:
Add-MailboxFolderPermission -identity “ManagingUser:\Calendar” -user “PersonalAssitant” -AccessRights Editor
