If someone leaves a comment on one of your Instagram posts, you can reply to them by following these simple steps:

1. Open the Instagram app and go to your profile.
2. Select the post with the comment you want to reply to.
3. Tap the speech bubble icon underneath the comment.
4. Type your response in the comment box that appears.
5. Tap "Post" to publish your response.

Your reply will be visible to the person who left the comment, as well as anyone else who views the post. Replying to comments is an important part of engaging with your audience on Instagram, and can help you build a sense of community around your content.