Change the Windows Automatic Updates Schedule on Server Core
Solution 1:
Assuming this isn't a domain machine you can configure through domain Group Policy, you should be able to connect a local Group Policy Editor mmc to the core machine from a full client and set the local policy that way (assuming 2012 still works as 2008 did):
http://blogs.msdn.com/b/neilhut/archive/2007/11/06/managing-local-policy-on-a-windows-server-core-installation-set-to-workgroup-config.aspx