In-place updating indeed does not work, and I wouldn’t expect too much from Chuwi on this unfortunately. You CAN however clean install 20H2 from a bootable USB-stick with the official 20H2 image from Microsoft, your license is automatically transferred: Download Windows 10 (microsoft.com). Be sure to backup anything important you have on there beforehand.
Note that a clean install will leave you with a couple of missing drivers, which you will need to source (or back up from your current install) and install yourself. See this thread for more info: We need LarkBox chipset drivers - MiniPC - CHUWI | Official Forum
Note of caution: I have done the above myself, and though everything seems to work ok, I am left with a couple of error messages in Windows Event Viewer: “Idle power management features on processor # in group 0 are disabled due to a firmware problem.". See also BIOS reset - Larkbox - #44 by Christy. Not sure if these errors are important, or if these weren’t even already present on 1909, but I can imagine they might have something to do with when the CPU ramps up or steps down. If any of you could confirm whether or not these Event Viewer entries are already present on your stock 1909 it would take that worry away- I would be interested to know.