I created a saved password via System ▸ Preferences ▸ Passwords and Encryption Keys ▸ File ▸ New ▸ Stored Password. How can I update it?
Expand "Login" by clicking the arrow beside it, right click on the password you wish to change and select "properties". Click the little arrow beside "Password" to show the password box, and click the "Show Password" option. You should now be able to see the old password, which you can overwrite with a new one.
My apologies, you can't edit it there ... although I'd swear I'd done it before. I can't edit my wireless password through the tool either. The upvotes on this answer should be reverted until I find the full answer.