You're the leader. You can edit your OP through your clan page here.
You do not need to make a new thread for a "dead" unofficial clan.
Being a leader should give you all these permissions. That's why you should be able to edit it yourself.
If the creator of the thread is not in the clan anymore, the only way to regain the power to edit the thread is to make a new thread, sorry.
That doesn't matter. If you can't see the edit button theres an issue. But you should look for it in the clan page. Take a screenshot of what you see.
Screenshot