First, hit the Office button(or File) and click the Account Settings button.
It will display the Account Settings window with Email tab open, if you have configured multiple accounts then all of them will be listed here. You will see the tabbed interface here, by clicking any particular tab you will see its relevant settings for the selected account.
The Change option lets you modify the settings of any particular email account, including the password.
At the very first step, you will see User Information, Server Information (Server Type, Incoming and Outgoing Server) and Logon Information( Email account and password). The password field is rght at the bottom.