To untrack a single file that has already been added/initialized to your repository, i.e., stop tracking the file but not delete it from your system use:
git rm --cached filename
Use --cached option to unstage and remove paths only from the index. Working tree files, whether modified or not, will be left alone.
To untrack every file that is now in your .gitignore:
First commit any outstanding code changes, and then, run this command:
git rm -r --cached .
This removes any changed files from the index(staging area), then just run:
git add .
Commit it:
git commit -m ".gitignore is now working"
To undo git rm --cached filename, use git add filename.
see https://stackoverflow.com/questions/1139762/ignore-files-that-have-already-been-committed-to-a-git-repository