There's way apparent in Explore to different which Xaps are only runnable and which are editable. Eventually we should make that searchable or apparent somehow from the thumbnail.
The way this is currently structured no Xaps are editable as they come from the explore page. The process of making Xaps and components public is limited to read (not write) privileges.
In your studio or home page editable Xaps and components will have a hammer icon, indicating you can edit them. To get editable Xaps you can coordinate with other users to have a Xap shared with write privileges.