I've just downloaded the new upgrade, but according to 'about Libre office' I'm still running the earlier one (despite clicking for 'replace existing').

I’ve tried downloading a couple of times, but still get the same. I’m on an iMac desktop running High Sierra, and have been using Libre Office for some years, originally with the early Mac (Pre Intel) PPC version on my old iMac G5, and now on my current iMac.

Did you restart LibO after downloading and installing the later version?