Using the default lemmy-ui you first have to find a post or comment that the user made in your community. Then you should be able to use the pop-up menu for that post/comment to unban them. It may be helpful to go to the user’s profile and search for a relevant post or comment there.

If you are comfortable using the API directly, you can send a POST request using a tool like curl or a browser plugin like RESTED. The site below provides a reference for formatting Lemmy API requests. You will need to get the community_id and person_id, and set ban=false.
https://lemmy.readme.io/reference/post_community-ban-user