You can Reply to a Post by selecting an individual Post.
1. Tap the Stream tab at the bottom panel to open up the Stream Page.
2. Locate the post you intend to reply to.
3. Tap on the reply icon in the lower right-hand corner of the post panel.
4. Type in your reply in the chat box in the middle of the bottom panel.
5. Tap the icon at the left of the bottom panel in order to attack a document, image, or video.
6. Click the "POST" icon at the right of the bottom panel to post the reply.