The problem: You made a post in a community, but you left some info out. Or possibly it's a really long post, and you mistyped the LJ Cut, and the whole thing is showing on your friends-page now and everyone will be mad. WOE! You know you can edit posts you made in your own journal by clicking Edit Entries on the main LJ menu, but that doesn't let you edit community entries. WOE!
What do I do?: Go to the individual entry that you made in the community, by clicking the "comment" link. No matter what style the community's comment pages are displayed in, somewhere on the page you will see a little blue pencil icon that looks like this: , or a link that says "Edit entry" -- usually it'll be the pencil. This will only appear on community posts if you are the person who made the post*. Click the pencil, and hey presto, you'll be able to edit the post. (This also works for posts on your own journal, of course.)
* (Or if you are the community owner. However if you're the community owner and you click to edit someone else's post, the only options you'll have are deleting it completely, or deleting it completely and marking it as spam, which alerts the Livejournal admin that someone is posting unwanted advertising messages in your community. You can't actually edit the text of someone else's post, even if you own the community.)