Should you need to undo a change that you have not yet committed (so that you can start over), you can do so on per-file basis by simply checking out the file again:
git checkout awkgram.y Undo changes to awkgram.y. There is no output
To start over completely, use ‘git reset --hard’. Note that this will throw away all your changes, with no chance for recovery, so be sure you really want to do it.