If you use the supplied (unwanted) password to just sign in, you'll see a row of buttons at the top of your screen, one marked PROFILE. You'll click MODIFY and then enter that nasty bad evil PW, and once you drill down into other PROFILE features you'll see where you can change the PW to whatever delights and amazes you.

Suggest you use upper AND lower-case letters, and at least one numeral. Experts recommend 12 or more characters in all.