If you don't, Windows 8.1 and Windows Server 2012 R2 will be put at risk.
Firms have until tomorrow to install a crucial Windows update or run the risk of leaving their devices vulnerable.
The update is for machines that run on Windows 8.1 clients, such as Windows Server 2012 R2 and Windows Embedded 8.1 Industry for devices, and was issued in April of this year.
If machines are not updated with this patch by August 12, all future updates including critical security fixes will not be detected by the devices, leaving them vulnerable to hackers. This is because the Windows Update service on the client or server will ignore any future updates, seeing them as 'not applicable'.
Firms using Windows 8 or Windows Server 2012 are not affected by this issue, however.
August 12 will also be the date that Microsoft will release its monthly software patches, known as Patch Tuesday.
The April update is called the 'Windows 8.1 Update', and this needs to be installed in order for organisations running on Windows 8.1 or Windows Server 2012 R2 in to receive a minor August update coming on August 12 as part of Patch Tuesday.