Connect to Office 365:

Set-MailboxCalendarConfiguration -Identity "<room name>" -WorkingHoursStartTime <start time> -WorkingHoursEndTime <end time> -WorkingHoursTimeZone "<timezone>"

Live example:

Set-MailboxCalendarConfiguration -Identity "Conf Room_One" -WorkingHoursStartTime 07:00:00 -WorkingHoursEndTime 20:00:00 -WorkingHoursTimeZone "W. Europe Standard Time"