Instagram: Hashtag change - so it works later
- Open the picture you want to Instagram and tap on the bottom right on the Button with the three dots.
- Here, select the "edit" Option, you can change the complete description, including Hashtags.
- Via the Button "Done" will be updated with the Hashtags.
- Note: on Android, that doesn't work, unfortunately. Here you can delete the image and re-adjust.

Instagram: Hashtag change
The full instructions can be in this article to read.
