Note to other people having trouble with a fresh install:
1: wait a couple hours. The below steps didn’t work for me after 20 min, but DID work when I woke up the next morning.
2: plug in monitor to the pi
3: enter root: type root as user and hit enter
4: type: os upgrade --version 4.20
5: give it like 20 minutes to complete
6: try going to http://homeassistant:8123/ on a computer or phone. If it doesn’t work, try another device.
7: Once logged into the UI, follow onlinegill’s instruction:
1) Go to supervisor
2) Click System (gear icon)
3) Under Host System, next to IP address click change. Expand IPV4 and change IP DHCP to static and under DNS server add google DNS : 184.108.40.206
4) Click Reboot
5) Go back to Supervisor menu and upgrade supervisor
8: Once supervisor is upgraded to most recent version, HA should show updated to latest as well.