Trying to upgrade my 2010 iMac OS system to Mojave.

My 2010 Mac is running on macOS High Sierra. I would like to upgrade to Mojave but was told it requires a certified third party download/possibly disk(?). BTW, my High Sierra is 10.13.6

Can anyone add any solid advice to this? If your from the Murrieta/Temecula/Lake Elsinore or even San Diego area in this community would greatly appreciate if you know of a location near here.

I was advised by Apple Support over the phone that ALL Apple stores in California are "shut down".

Thanks.