There's two ways:

1. Modify the existing commit.  `git checkout patch-1`, make your changes, `git commit --amend GPU/Common/VertexDecoderCommon.h`, `git push origin patch-1 -f`.

2. Add additional commits (better for further changes, not removing things.)  `git checkout patch-1`, make your changes, `git commit GPU/Common/VertexDecoderCommon.h`, `git push origin patch-1 -f`.

I think there's a page in the wiki on the right about it too.

-[Unknown]