How do I add, edit or delete the caption of an existing post in instagram ?
You can add a caption to a photo or video you've already shared, or
edit or delete the caption you originally included. To add, edit or
delete a caption:
- Tap
(iOS) or 
(Android) above your photo or video - Tap Edit
- Edit, delete or add a caption, then tap Done
When you tap 
below a post, the word Edited will appear above captions that have been edited.
below a post, the word Edited will appear above captions that have been edited.