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.