1) To edit the web gallery, change the images in the gallery that you are sharing. Then use the 'Create' menu option in Web Publishing to overwrite/update the website.
2) Use F->Web Sharing->Create and select the 'select upload' button to select the gallery you wish to use to overwrite the current gallery.
Removing or replacing the web gallery
If you don't want any published, then delete the images in the gallery, or create a new empty gallery called (for example) 'EmptyGallery' to sync with the web, you can then hide this gallery by disabling it with F->Portfolio->EmptyGallery
Once you have created a suitable empty gallery, or emptied the currently web synced gallery use
F->Web publishing->Create and select the 'select upload' button' to select the new Empty Gallery.