To retag a question don't click on `edit` link. Click on `retag` link. 

From [retag privilege][1] 

> **How do I retag a question?**
> 
> Click the `retag` menu option under the post.

![enter image description here][2]


  [1]: https://stackoverflow.com/privileges/retag-questions
  [2]: https://i.sstatic.net/djz5R.png