TLDR - You are safe and don't need to do anything, as you run windows updates.
Visit https://assured-id-root.digicert.com/ if the webpage says you are ok, you are.

In detail:
Generally you can trust that you have the certificate if you use windows updates.  You (or an IT department in your organisation) would have to disallow the update for that not to be the case and it would be manifesting in security issues as you browse the web or try to install other vendors software. This update keeps you safe on the internet, no competent IT manager would disallow it.

If you do need to update, visiting the digicert website https://www.digicert.com (note the S in https as you are browsing to the secure site which is signed against the root certificate) will prompt a security warning and you will be asked to install it.

If you do wish to download the root certificate manually you can get them from https://www.digicert.com/digicert-root-certificates.htm you need the first one on the page (DigiCert Assured ID Root CA).

If you manually wish to look into the certificate manager locally you can run certmgr.msc from a command prompt, a run box, the search box on the start menu or you can look at the certificates installed in a web browser. For example in internet explorer settings you can see a certificates option in the privacy tab.

You can also right click on any SDL Installer and choose properties, then go to the digital signature tab (if there is not one, beware of the installer as it may have been tampered with, delete it and re-download it from OOS). On the tab you can see the validity of the signing, if you choose the signature and click details you can see that the certificate chain is valid (there will be errors or warnings if something is amiss).