Historically, uncertainty about who is the rightful leader has often led to violence. While I think the most likely outcome for the coming election in the US is a peaceful transition from Trump to Biden, the range of possibilities is wide enough that it's worth making some preparations for worse outcomes.

The most likely risk to me seems to be supply chain disruptions. If things go poorly, staying home is probably going to be your best option, and you want to have whatever supplies you need on hand so that you don't have to go out. This is always good policy, but is especially important if there is a potential disaster you can see coming. Make sure you have enough food, refills on medicines, and spares for critical items (glasses, medical equipment). If there are things that you usually purchase in a "just in time" manner, buy ahead. Think through things you might be going to want and consider getting them now instead. Check in with friends and relatives and make sure they're in an ok spot.

I think there is also some risk of rioting and fires, especially on election night or perhaps after a court verdict is announced. If you have a car, making sure you have a full tank of gas is worth it. Aside from that, however, I think planning for riots is not that different for planning for supply issues. If you would be considering participating in riots, you would probably also want to plan in advance for any gear you might want.

Someone who was much more risk averse than I am, or who anticipated a much worse outcome, might consider making plans to be in a remote area, or outside the country. I'd recommend against buying firearms: there's a lot of practice that goes into using them well, and if you don't know what you're doing you're probably going to make things worse.

As someone who would like to see Biden elected, and expects you do as well, I think the most important thing to do is to vote. A clear victory is hard to dispute.