How to select a specific subsysmtem on WSL?

Hello I have the following installations on my WSL. I want to 1)select and 2) update the following subsystem: Ubuntu-18.04

Update:

1. I can run wsl -s Ubuntu-18.04 to select some release

You can use the -d/--distribution <distroname> option to run a particular instance. So wsl -d Ubuntu-18.04 will start that distribution and allow you to update. Better yet (and undocumented), wsl ~ -d Ubuntu-18.04, since that will start you in your home directory.

The -s that you mentioned is short for --select-default. This changes the default instance so that the next time you run wsl without a -d/--distribution option, the default runs.

There are quite a few useful options documented here, starting with -h/--help which will give you a short overview in the console.

Also, for updating the instance, you can do it in a one-liner with:

wsl -d Ubuntu-18.04 -u root sh -c "apt update && apt upgrade"