This was a .tex document that I had been editing in a text editor, not a Google doc. I tried context menus on the file in both Windows and the Google drive webs

This was a .tex document that I had been editing in a text editor, not a Google doc. I tried context menus on the file in both Windows and the Google drive website to no avail, could spot any other relevant buttons in the rest of the Google drive websites interface either. ---------- Just tried it again now, and found the option on the website as 'manage versions' under the context menu of a file (no idea how I missed it the first time). Still had to download the old version and replace the file manually rather than an explicit revert button.