Run this command in your terminal in the directory the iso was downloaded to verify the SHA256 checksum:
echo "e2ce1771e352b04cfdef5bf6583f6a7d62b1f2967903fae512506d18d251a434 *ubuntu-20.04.2-desktop-amd64.iso" | shasum -a 256 --check
You should get the following output:
ubuntu-20.04.2-desktop-amd64.iso: OK