Run this command in your terminal in the directory the iso was downloaded to verify the SHA256 checksum:
echo "d1f2bf834bbe9bb43faf16f9be992a6f3935e65be0edece1dee2aa6eb1767423 *ubuntu-20.04.2-live-server-amd64.iso" | shasum -a 256 --check
You should get the following output:
ubuntu-20.04.2-live-server-amd64.iso: OK