Catalina update from Mojave: how to continue the download from where it was?
Out of curiosity, considering you asked this 5 months ago, have you still not updated yet?

Nevertheless, here's my answer:

As far as I'm aware, if the connection is lost, the software update is cancelled, and there usually is no easy way to restart from where it was lost. A cancellation by definition usually involves the loss of progress.
I suppose, however, that there may be some obscure method by which this might be possible (I see a few comments which have various suggestions), but to be honest, its far more reasonable to simply redo the update than to take the risk to tamper with the software which runs your Mac, especially during a major MacOS update. Its simply not worth taking the risk to save 30 minutes.
I'm sure many people have had this sort of issue and wanted to do this before and if it was that simple Apple probably would've already made it so that you could start off from where you left. This suggests that it is not, so stay on the safe side.

Conclusion


So conclusion - maybe there is some complicated method which would do this, but my advice: don't risk it, just take a few extra minutes and you'll be safe, rather than sorry.