What version of Firefox Focus do I have?

How do you update any other app? [https://apps.apple.com/us/app/firefox-focus-privacy-browser/id1055677337 It's version 155, you're OK].

i don't know

Focus iOS does not ship during v159 — its releases are logged e.g. in github.com/mozilla-mobile/firefox-ios/releases?q=Focus that shows which versions are available. When there are updates, you App Store will offer that and install it.