Haha It's a bit fiddly right now because the website's problems have knackered it but, PSA I guess for anyone else looking to update their signature:

It's in settings (top left tab) > Edit Signature ( On the Left column) > Then just type whatever into that box.

If you try to preview your signature it will delete itself, similar to previewing a post in Go Advanced. But you can check your siggy by saving it and doing back into Edit Signature again. Should be above the text box thing.