You can only change your email from within the application.
To do so please take the following steps:
1. Log into the account that you wish to change the email for
2. Tap on 'Menu'
3. Tap on the 'Gear'
4. Tap on 'Account Info'
5. From within this menu you can change your Email Address and Password
If you do not have access to your email account and are attempting to access your PK account to change your email, you will need to contact your email provider (e.g. Gmail, Yahoo, etc.) to regain access to your email account.