@citizenjoe said:
Thanks Lucas. My question was about El Capitan
I personally updated one of the two trashcans from El Capitan to Mojave. There are instructions in this article in the section "How to install an update to macOS in High Sierra or earlier" on how to update that generation of macOS to a newer version.
Also, though El Capitan is not officially within our list of supported operating systems, Isadora 3.1.1 might work fine on it. My laptop was running Isadora 3.0.8f12 on El Capitan without any problems. Testing is key though. Ask them to boot up the machine and hop on a zoom call with you so that you can remote access the machine through zoom in order to install Isadora 3.1.1 and make sure everything works alright.