Sometimes a remote host or your ISP is temporarily misconfigured with regard to IPv6.
In this case,
apt update simply hangs, for example at
Connecting to mirrordirector.raspbian.org (2a00:1098:0:80:1000:75:0:3) Connecting to archive.raspberrypi.org (2a00:1098:0:80:1000:13:0:5)
You can temporarily force
apt to use IPv4:
apt -o Acquire::ForceIPv4=true update apt -o Acquire::ForceIPv4=true upgrade