I know that my question seems to be quite stupid, however I would like to know how I can edit a website. I have created websites before and edit the files using FileZilla. Can I use the same method even when the website is live?

Also how do I get to know the server and password?

Thank you and again sorry if my question seems stupid.

You can use FTP to download the files to work on them. Once you're finished FTP them back to the site. Any downtime to a particular page or the site in general should be pretty minimal.
As for the server and password the site administrator will need to give that to you.

As hericles mentions...when you upload the file(s) you edited on your computer back to the hosted location, you will overwrite the existing file(s). The next time that file is requested by a browser, the newer version of content will be seen by your visitors.

Rather than thinking about the site as "live", consider it as the production copy of your site. What you edit is you local copy you use as development and/or validation. When you have validated that your code is working, you move it to production.

Generally if you site is hosted, you can just log onto the admin panel and manage your username and password for the FTP service.

For a live site You must be having admin Access you can Simply Logon and make the Required changes in the coding.