First of all, it's important to note that you will only have an account and password if you are using a G1 WiFi Gateway in conjunction with your sensors. If you are only using SensorPush sensors directly with your mobile device(s) via Bluetooth, there is no need for an account, nor is there a way to create one.
With that aside, there are two ways to change your password:
IN THE APP:
1.) Open the app, and from the main screen (the screen with the word "Devices" at the top) select the button in the top left to open app settings.
2.) Once in app settings you'll notice a section called "Account." If you are already logged in, select the button to LOGOUT.
3.) Tap the LOGIN button.
4.) Type your email address in the email field and then click the "Forgot Password" link.
This will send an email to your email for changing your password.
IN THE WEB DASHBOARD:
You can also change the password for the account using the web dashboard for the Gateway: https://dashboard.sensorpush.com
Once you log in, go to the settings icon in the upper right where you will find the option to change the password.