Currently, the only way to change your PowerCenter and Stryd app password is via http://www.stryd.com/powercenter. Sign out from the application (top right corner), which will take you to the login screen:
Select "Forgot Password", which redirects you to another window asking for your email address. Once you enter your email address, you will get an automated email to reset your password. Follow the link in the email and enter your new password.
If you want to change your username, or no longer use your Facebook, Twitter or Google account, contact firstname.lastname@example.org.
If you have any problem, contact email@example.com.