It is pretty simple. On the top right of any Trulia page you can see your name when you're logged in. there is a drop-down arrow right next to your name. Point to it and there will be a drop-down menu - Click on My Profile - On the right hand side under the green Trulia banner/ header click on "Edit my profile" - Your profile page will open up with the "Personal Information" field open, but right above that there is the Email and Password category, which is not opened automatically, so you might have overlooked it. To the right of the category header there's a " + " sign (almost transparent), click on the " + " sign and "Email and Password" field will open up.
Here you can change your email address and/ or your password - Click on Save.
Edit my profile
Email and Password
Change and Save