Sorry for the inconvenience here! There is a problem with versions 3.4.0-nexthw1, 3.4.2-nexthw1 and 3.4.3-nexthw1 where the upgrade process is not working quite right.
To recover from this situation, open the Terminal app and at the prompt, carefully type the command:
sudo ostree admin upgrade
It may ask for your user password (which will not be displayed as you type it in).
It will then go and download the update, so please be patient until the process finishes, it will say Bootloader updated; bootconfig swap: yes deployment count change: 1
At that point, reboot your computer and it will load the new version of Endless OS. Automatic updates will work from this point onwards and hopefully if you then install the latest Steam version it will work fine.