As per Old Teck Bloke's video on Youtube:

https://www.youtube.com/watch?v=IzTtZyzSkv8&t=53s

1. Get https://slakfinder.org/slackpkg+.html and install it using installpkg
2. Edit /etc/slackpkg/slackpkgplus.conf to use alienbob and alienbob restricted
3. slackpkg update gpg to get keys from alienbob
4. slackpkg update to update software list
5. Follow normal slackware update proceedures as per slackware.md in Dropbox

For multilib:

1. Following instructions in /etc/slackpkg/slackpkgplus.conf, as root execute the script in /usr/share/doc/slackpkg+-version/setupmultilib.sh

This was quite slow and it gave up trying to download qt5-32compat and I had to manually install that using slackpkg.

Steam:

Lookking at alien, there is now a package for steam. I installed that using

slackpgk install steamclient