I needed to change permissions on who could access the CEO’s calendar as his personal assistant was going on a month-long vacation and the backup needed the same permissions.

Connect to your Office365 environment using Powershell
Open Powershell as an administrator
$LiveCred = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://ps.outlook.com/powershell/ -Credential $LiveCred -Authentication Basic -AllowRedirection
Import-PSSession $Session
Login with an O365 admin account

List access on a specific account
Get-MailboxFolderPermission -Identity user1@domain.com:\calendar

Add access to a specific user
Add-MailboxFolderPermission -Identity user1@domain.com:\calendar -user user2@domain.com -AccessRights Editor

Remove access to a specific user
Remove-MailboxFolderPermission -Identity user1@domain.com:\calendar -user user2@domain.com

Available Roles

Owner
PublishingEditor
Editor
PublishingAuthor
Author
NonEditingAuthor
Reviewer
Contributor
AvailabilityOnly
LimitedDetails