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
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.
Make sure to commit all your important changes before running
git add .
Otherwise, you will lose any changes to other files.
Please be careful, when you push this to a repository and pull from somewhere else into a state where those files are still tracked, the files will be DELETED