I can’t update iOS

If you're trying to upgrade iOS, you might already be running the latest version that your iPhone will support.

What iPhone do you have?

iPhone 6

What are you trying to accomplish?

If you need to see the list of trusted certificates, then in Finder -> Go enter (without the TrustStore.html at the end)

/System/Library/Security/Certificates.bundle/Contents/Resources/

you should see the file TrustStore.html. If you open it via Safari, you should see a list similar to