It’s true: You can add mentions to an Instagram Story even after it’s posted. And it’s pretty simple.
You can’t edit a Story after it’s been posted — meaning if you want to add text, music, or images after the fact, you’ll have to delete the post and repost it as you wish. But if all you need to do is add a mention after you post your Instagram Story, and if you don’t mind that their username doesn’t appear in the story, try this.
Step 1: Add to your Instagram Story
You probably already know how to do this, but click on the [+] sign in the lower middle of your Instagram home screen, click your profile picture in the upper left corner, or slide your screen to the left. This will allow you to add to your Instagram Story.

Credit: Screenshot on Instagram
Step 2: Post to your Instagram Story
Take a photo or video, add your text and anything else you want to add, and add it to your Instagram Story or your Close Friends Story.

Credit: Screenshot on Instagram
Step 3: Navigate to Add Mentions after posting your Instagram Story
Once your Instagram Story is posted, click the three dots in the lower right corner of the screen that says “more.” This will show you a screen with eight options, including “Delete,” “Highlight,” “Save,” “Share as post,” “Send,” “Add Mentions,” “Boost story,” and “Story settings .” Click “Add Mentions.”

Credit: Screenshot on Instagram
Step 4: Select the people you want to mention in your Instagram Story after it is posted
Scroll through the people you serve on Instagram, or type their username in the Search bar. Click on the name of the account you want to mention in your Instagram Story, and the circle to the right next to their username will be filled with a blue check mark. Click “Add,” and the account will be tagged in your Instagram Story.
As a reminder: You will not see the account added and, if you mention someone in a story, they will receive a notification.