I have made some changes to a file which has been committed a few times as part of a group of files, but now want to reset/revert the changes on it back to a previous versio
I had the same issue just now and I found this answer easiest to understand (commit-ref is the SHA value of the change in the log you want to go back to):
git checkout [commit-ref] [filename]
This will put that old version in your working directory and from there you can commit it if you want.