If you are not sure, then its best to leave it run. My "assumption" would be that it is safe to cancel during the download period. I am actually not familiar how Linux Mint GUI operates, as I am using the upgrade and update commands on an Archlinux* system in the terminal. So I can see exactly what it is doing, and when it enters into installation phase after downloading all necessary packages. During installation I wouldn't cancel the process. My suggestion was for you to look into.
*EndeavourOS
Edit: Typo