Some older operating systems may not support security updates. This guide shows how to manually fix the issue of having an untrusted or old root certificate. 

1. Download the Root Certificate. You can do so, for example, by accessing this Sectigo website. 

2. Double click on the Certificate. This will open up an Add Certification dialog. 

3. Choose System in the bottom right menu. Then, click on Add.

4. Enter your admin password and click on Modify Keychain. You might be prompted to use your fingerprint if you have a different configuration - make sure to switch to admin/password authentication.

5. At the top menu choose Window > Keychain Viewer. You should still have the Keychain Access application open.

6. Select System on the left and search for “Sectigo” The Sectigo Certificate will show up - after that, double-click on that line to open its settings.

7. Click on the arrow to the left of the Trust dropdown.

8. Change the first menu from “Use system defaults” to “Always Trust”.

9. Close that window and enter your Admin password when asked.

And that is it! You now have an updated public certificate for Sectigo.