In my experience, not pushing it makes them want to try it themselves at some point. I guess you need to take care of their computer frequently enough, and are probably annoyed by Windows shitting its pants every time again. Don't make any drama out of it, just point out how ridiculous it is that Microsoft cannot manage to build something that allows running two simple programs without breaking or nagging the user so often. They know that you use something else with which you're happy with, and at some point they will become curious and ask wheter they can have it too. At that point do not promise much, say that it works a lot better but is also a lot different and sometimes a bit quirky. Do not rush it now, let them simmer in their curiousity. At a fitting occasion tell them very briefly about foss and how it is not a closed thing pushed by a corporation onto individuals to funnel data. When they ask if they can try it, tell them they can but it takes a bit of getting used to. Buy a new SSD, and safely store the previous storage in a anti static bag, exclaiming that everything is on there and cannot get lost due to linux. Set everything up with a dead easy DE, give clear tour of how stuff works. With this tactic, they want to get it to work by themselves, and are prepared to learn that some things work differently. It becomes an adventure that is totally revertable if it doesn't work out. In contrast to when you want to force the change and they use everything as a reason to be unhappy about it.