The mod tools are unfortunately pretty poor on Lemmy. For adding/removing moderators via the GUI the person must first post/comment in that specific community. You can then via the context menu of that post/comment add someone as a mod.
The alternative is to interact with the Lemmy API directly via a script.
I've added myself as a moderator, although the whole admin team may operate as moderators, similar to [email protected].
If you got additional changes you want to make to the community, e.g. add additional rules like make it explicit that only you can post, or add a banner to the community you should do it now before you're removed as a moderator. Otherwise you can always DM me/the admin team if you want to make changes to it.
Edit: As Blaze pointed out, you can use alternate frontends like https://t.programming.dev/ to gain additional GUI mod tools