How can I change a secret Gist to public?

I upload a Gist code and set it to be secret.
Now I want to change the gist to public, but I can't find how to set it publi

Since May, 9th 2014, it is possible: "Change the visibility of your Gists"

You can change the visibility of your Gists whenever you want.

When editing a Gist you'll now notice a new option to toggle the visibility between Public and Secret.
The URL for your Gist will never change, just its visibility.

As commented by GiDo though, since 2016:

you can only make public a gist that was previously private.
When it is public it will stay public.

Update:

You can still convert a secret gist to a public gist by first selecting "Edit" -> "Make Public". This feature has been partially rolled back, however, since 12/15/16 and you will no longer be able to switch it back to secret.

From the docs:

After creating a gist, you cannot convert it from public to secret.