Follow these instructions 
User A (Mike) must share his calendar with User B (Tom). Tom needs to accept the calendar share and by default he'll have Viewer rights to Mike's calendar.
Open up PowerShell and enter:
Code:
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session
To view the existing permissions on Mike's calendar, enter:
Code:
Get-MailboxFolderPermission -identity "Mike:\Calendar"
You'll see something like this:
Code:
RunspaceId : 12345678900987654321
FolderName : Calendar
User : Tom
AccessRights : {Reviewer}
Identity : Tom
IsValid : True You now need to remove the existing Reviewer permissions as follows:
Code:
Remove-MailboxFolderPermission -identity "Mike:\Calendar" -user "Tom"
You can now add Editor rights as follows:
Code:
Add-MailboxFolderPermission -identity "Mike:\Calendar" -user "Tom" -AccessRights Editor
Now if you re-run this command:
Code:
Get-MailboxFolderPermission -identity "Mike:\Calendar"
The response should be something like:
Code:
RunspaceId : 12345678900987654321
FolderName : Calendar
User : Tom
AccessRights : {Editor}
Identity : Tom
IsValid : True Job done, Tom can now edit Mike's calendar.