Open Lemmy in a browser, not an app. Click on the pencil symbol in the upper right corner of the screen. Here’s an example:

Edit: Looking at your profile, you aren’t the moderator of any communities. You need to be a mod before you can make edits. Get in touch with the admins at the instance where you created your community and ask them to make you the moderator.

On lemmy.world you click the little clipboard looking thing underneath your community’s name on the right side of the screen.