You can not change your password in your admin area. What you need to do is on the Shopstar log in screen click on the "Forgot your password?" link below the login button. This will bring up the Password Recovery page and will ask you for your email address. Enter your email address that you used to sign up your shop and click on the "Send me reset password instructions" button. This will email you your instructions to reset your password.
Once you've received the email, click on the "Set my password" link in the bottom of the email. This will take you to the "Change your password" screen. Enter your new password and click on "Reset my password" button. Your password has now been changed.
Interesting reads on choosing good passwords and keeping your passwords safe: