Hello!
You need to use the itch.io app to update the game files, you can't do it in the browser.
https://itch.io/app
Thanks; I did use the itch.io app. The Download button informed me it was already installed (1.08 version), when I found the 'Check for updates' menu option (right clicking on the Outpost Glacier entry IIRC) and selected it, it seemed nothing was done and I still had the 1.08 version. Then I created this original post asking how to proceed; on checking back after you replied I found and clicked 'Check for updates' again, and this time I was told it was up to date and indeed the correct 1.09 version is what was run. So perhaps there was some update feedback I had missed, or wasn't presented where I could see it, or I just needed to wait for the update to complete.