Nothing tricky. Just replace all occurrance of bionic with cosmic in /etc/apt/sources.list and do a apt full-upgrade. For details, see Upgrading from Ubuntu 18.04.

But what worth noting is that after the upgrade /etc/sysctl.d/10-ipv6-privacy.conf no longer works as expected. RFC 4941 is not in effect anymore. According to What about ‘net.ipv6.conf.default.use_tempaddr’ and ‘net.ipv6.conf.all.use_tempaddr’?, this should have never worked (and not considered as a BUG) but weirdly it did work in Ubuntu 18.04.

To enable RFC 4941 on eth0 (again), I created another file under /etc/sysctl.d/ with:

net.ipv6.conf.eth0.use_tempaddr = 2

Not sure if related or I just did not notice, but after upgraded to Ubuntu 18.10, the wireless interface shows up in ifconfig.

Now the box looks like this:

# neofetch
            .-/+oossssoo+/-.               root@localhost
        `:+ssssssssssssssssss+:`           ---------------
      -+ssssssssssssssssssyyssss+-         OS: Ubuntu 18.10 aarch64
    .ossssssssssssssssssdMMMNysssso.       Host: Phicomm N1
   /ssssssssssshdmmNNmmyNMMMMhssssss/      Kernel: 4.19.6-aml-s9xxx
  +ssssssssshmydMMMMMMMNddddyssssssss+     Uptime: 2 mins
 /sssssssshNMMMyhhyyyyhmNMMMNhssssssss/    Packages: 512 (dpkg)
.ssssssssdMMMNhsssssssssshNMMMdssssssss.   Shell: bash 4.4.19
+sssshhhyNMMNyssssssssssssyNMMMysssssss+   Terminal: /dev/pts/0
ossyNMMMNyMMhsssssssssssssshmmmhssssssso   CPU: (4) @ 1.512GHz
ossyNMMMNyMMhsssssssssssssshmmmhssssssso   Memory: 167MiB / 1843MiB
+sssshhhyNMMNyssssssssssssyNMMMysssssss+
.ssssssssdMMMNhsssssssssshNMMMdssssssss.
 /sssssssshNMMMyhhyyyyhdNMMMNhssssssss/
  +sssssssssdmydMMMMMMMMddddyssssssss+
   /ssssssssssshdmNNNNmyNMMMMhssssss/
    .ossssssssssssssssssdMMMNysssso.
      -+sssssssssssssssssyyyssss+-
        `:+ssssssssssssssssss+:`
            .-/+oossssoo+/-.