hi nitin,
generally the way Netlify works is that we trigger a deploy from a git repo that is hosted somewhere - such as github, bitbucket, gitlab, so then the content would be in that repo. The general workflow would be to edit, add the files to git, commit, push, which should trigger a redeploy.
if you did a manual (drag n drop) deploy, then no repo is involved. If you no longer have the files that you deployed via drag n drop, you can download a copy of your site:
to make changes, you’d either drag n drop “over” the existing one, or, you’d set up a git repo, and commit/push to trigger a redeploy.
Does that make sense?