The car says update is available (I'm currently on 2019.20.2.1) and that I should get on WiFi to download new update.
If this is an accurate description of the prompt, then it sounds like the update has
not been downloaded, contrary to what others have been saying. Details seem to be vague, but I've seen some claims that SR and SR+ Model 3s will download non-critical updates only over WiFi, whereas higher-trim Model 3s are more willing to download updates over LTE. As most Model 3 owners until recently have the higher-trim versions, we might not see the prompt to connect to WiFi very often, even if our WiFi connections are poor or non-existent.
If my hypothesis that you really do need to connect to WiFi is correct, I'd urge patience. Be sure your WiFi signal is good. (You can try using "ping" from a computer on your home network to ping your Model 3. Be sure you have few or no dropped packets over a run of at least 100 pings.) If you see a lot of dropped packets, try moving your car closer to your WiFi router, if that's possible; and if not, look into buying a WiFi signal repeater or extender to improve signal strength in your garage or driveway. Alternatively, you could try setting up your cell phone as a WiFi hot spot, connect the car to it, and leave it near the car for a while -- but this may incur charges from your cell phone service provider if the download brings your data total over your monthly data allotment. Connecting to a free public WiFi hot spot might also work. If nothing else, Tesla stores should have WiFi, and Teslas should connect to those WiFi networks automatically. In any event, once you get a good WiFi connection, you may need to wait a while. I know of no way to force the car to download an update at a specific time; the car will do it whenever it wants to, which could be immediately, an hour after you connect, a day after you connect, or whatever.
All of this assumes, of course, that you've accurately reported the message and that I'm interpreting it correctly. As others have said, it's possible the car has already downloaded the update and is just waiting for you to approve its installation. In my experience, though, the car is pretty in-your-face about this. It pops up a big window asking to install the update; there's nothing subtle or unclear about it. If you've seen and dismissed that window, you can get it back by touching an icon at the top of the screen. IIRC, it's a timer/clock icon.