Instagram allows you to add a caption to a photo or video before sharing it. If you do not to add a caption while sharing your photo, then you can add or edit the caption. To add or edit your caption, open "Instagram" app on your mobile. Tap "..." (3 horizontal dots) on iPhone or "3 vertical dots" on Android phone located below your photo or video. Tap "Edit" button and edit or add your caption to the photo and then tap "Done" button. You will see a word "Edited" above captions that have been edited when you tap on a photo or video.