How can I update a CSV file in netsuite ? I need to add more columns and update the existing file.

CSV is basically a text. You can apply any string functions to it

Do you have the code for that? It's not happening

You first load a csv file from FileCabinet and you get it as text:

const fileString = file.load({
    id: 123456,
}).getContents();

Then you do with the string whatever you want, then you save it :

const fileObj = file.create({
    name: `${fileName}.csv`,
    fileType: file.Type.PLAINTEXT,
    contents: someAnotherText,
});