From the Admin Dashboard, click on your name and select Change Password.
Go to the section Change your PIN, it's on the right of the screen.
Enter your current Password and 4 digits for your new PIN.
Click the Change PIN button. You can now log onto the App with your new pin number.
Note: If you are doing this on a phone or tablet, you will see the Change the Password section first, ignore that and scroll further for the Change your PIN section.