can I for example upgrade the OS?

I mean, technically, sure.

But what I suspect you’re really asking is “has someone else done the legwork to create a stable build of a modern version of android for what is probably a very underpowered device since it’s in a vehicle.”

The reality is you would probably have to dig into the hardware, find appropriate drivers, and then dig into the AOSP and build a stable Android build for it from there. And depending on it’s age, you might only be able to upgrade it to a certain point and not all the way to current Android. (For example, LineageOS no longer directly supports Google Nexus devices because their hardware is simply too old to work with the new versions of Android)

Because sadly, the likelihood of someone already having done it for an obscure vehicle head unit is less likely than, say, a flagship phone that tons of people own. It’s not impossible, someone may have already done the work, but I’d wager on that not being the case, because I’ve had just slightly unpopular phones that there are no LineageOS builds for, for example.