source: https://docs.microsoft.com/en-us/powershell/exchange/connect-to-exchange-servers-using-remote-powershell?view=exchange-ps

PowerShell commands

PowerShell (Run as Administrator)

Set-ExecutionPolicy Unrestricted
$creds = Get-Credential
$sourceone = New-PSSession –ConfigurationName Microsoft.Exchange –ConnectionUri “https://outlook.office365.com/powershell-liveid/” -Credential $creds –Authentication Basic -AllowRedirection
Import-PSSession $sourceone

For all shared mailboxes:
Get-Mailbox –RecipientTypeDetails SharedMailbox | Set-MailboxRegionalConfiguration –Language “en-US” –TimeZone “Central Standard Time” –DateFormat “M/d/yyyy” –TimeFormat “h:mm tt”

For a single shared or user mailbox:
Get-Mailbox –Identity sharedmailbox@sourceonetechnology.com | Set-MailboxRegionalConfiguration –Language “en-US” –TimeZone “Central Standard Time” –DateFormat “M/d/yyyy” –TimeFormat “h:mm tt”

For *all* mailboxes:
Get-Mailbox | Set-MailboxRegionalConfiguration –Language “en-US” –TimeZone “Central Standard Time” –DateFormat “M/d/yyyy” –TimeFormat “h:mm tt”

Remove-PSSession $sourceone