How to remove a file from the index in git?

You want:

git rm --cached [file]

If you omit the --cached option, it will also delete it from the working tree. git rm is slightly safer than git reset, because you'll be warned if the staged content doesn't match either the tip of the branch or the file on disk. (If it doesn't, you have to add --force.)

This should unstage a <file> for you (without removing or otherwise modifying the file):

git reset <file>

git reset HEAD <file> 

for removing a particular file from the index.

and

git reset HEAD

for removing all indexed files.