var fs = require('fs');

fs.readFile('index.txt', 'utf-8', function(err, data) {
    if (err) throw err;

    var newValue = data.replace(/email/gim, 'name');

    fs.writeFile('index.txt', newValue, 'utf-8', function(err, data) {
        if (err) throw err;
        console.log('Done!');
    });
});