One approach is to use GitHub, click on the history next to the number of commits, and click on a specific commit, and then click the browse button to browse the files that were in the repository at that point in time. Then you could copy code down from the website and paste it into your Eclipse.
Another approach would be in Eclipse to right click on a file and do "Compare with" previous revision, and when you find the version you're looking for, you could right click and do "Replace with" that version.
If you really want to undo a specific commit and the exact set of changes in that commit, then you can use the git revert
command from the command line, and you specify the long unique identifier (e.g. ed2f64...) for the commit you want to undo.