First, you need to go to the setting. you do this either by swiping left on your device or tap the 3 horizontal lines in the top right corner.
Once you have enter settings you will need to unlock your account. you do this by tapping on the unlock account. you will then be asked to confirm your original password.
Now the option to change your password/pin should be highlighted.
Tap the option you want to change and you are done!