git update-index function has several option you can find typing as below:
git update-index --help
Here you will find various option - how to handle with the function update-index.
[if you don't know the file name]
git update-index --really-refresh
[if you know the file name ]
git update-index --no-assume-unchanged <file>
will revert all the files those have been added in ignore list through.
git update-index --assume-unchanged <file>