You can change your account information under the Settings section in the app.
 
If you are currently on the Now Playing screen touch the back arrow in the upper left (near the Pandora icon). Then select the Settings tab.
 
Next, touch Account and you will be able to change all your registration information, including your email address and password. 
 
Make sure to hit ‘Save’ when you are finished.
 
When it asks for your password to save any changes, please enter your OLD password. Then the new one will take effect.