long download on imac loses wifi connection interrupts ventura upgrade fromhigh sierra. What to do?

If you have no access to an Ethernet connection, you could try upgrading while in Safe Mode to make sure nothing installed is interfering with the download.

How to use safe mode on your Mac - Apple Support

Using Wi-Fi to download upgrade files will often corrupt those files and they will fail to install as they should. The reason is that Wi-Fi is often too slow for these very large files. Instead, use a wired Ethernet connection if possible and it should work just fine. Otherwise, you may have to do several Wi-Fi downloads to get a clean one that works.