Hi, @Fabien MILOCHEVITCH
You may run this command to assign owner permission on the calendar to other employees to enable them to manage the appointments.
Add-MailboxFolderPermission -Identity ******@contoso.com:\Calendar -User ******@contoso.com -AccessRights Owner
The new employee may update, delete the existing appointments and create new appointments in the calendar.
However the owner of the existing and newly created appointments would remain to be the leaving employee since he or she is the owner of the mailbox and calendar.
If you are using Outlook, the detailed steps to open the shared calendar are introduced in this link: Open another person's Exchange Calendar
If you are using OWA, the calendar can be accessed via Calendar>right click on "Other calendars">select "open calendar">enter the leaving employee's mailbox in "From directory" field.
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