Once you've logged into BC, click your name at the top right of BC then click Change Password:
You will be taken to the following page:
All you need to do here is enter the password that you're currently using into the first of the 3 fields, and then whatever you want to change the password to into the 2nd and 3rd fields. Please make sure you are abiding by the password rules displayed on the page.
Once you've done that, click on Change Password and you'll get a confirmation page from BC.