How do you change Password Keychains?

Generally speaking, if you re-type the correct password the browser will ask you if you want to save the new password or keep the old one.

Since it's not: In the Utilities folder (in Applications) open Keychain Access. Search for an Amazon (or whatever) keychain item (it'll generally be in the "login" keychain, which is the default). Double click to open it, check the box and authenticate to show the password, re-type it, click save, and you're done.