If you just want to change your password for some reason then all you have to do is go to "Account Settings":

Then underneath "General Settings" in the "Password" column on the right you can click on the "Edit" link. Note you must know your existing password for this to work.

Optionally, you can turn on the "Automatically log me in from links in email notifications" toggle button to be able to click on an email notification link and have the system log you in without asking for your password: