Hopefully, you have a copy of all your stories saved somewhere, and not just existing on Medium. If not, copy and paste the text of story to your computer. Then you can find the command to Delete the old story under the “…” menu on a computer. On the mobile apps you have to go to your profile, then to Stories and find the story, then select delete.

Then you will need to create a new story and publish it. There is some controversy among writers about “republishing” old stories without changes. Some people do. Others think it is wrong. Others consider a story a new story if they make significant changes to it. It’s really your choice.