If you know your current password and want to change it, do it from Settings. If you've forgotten it, use the reset flow instead — see Resetting your password.
How to change it
Go to Settings.
Find the Update Password section.
Enter your current password.
Enter your new password, then confirm it.
Click Save.
A "Saved." confirmation appears when it's done.
Password requirements
At least 8 characters. Beyond that, length matters more than complexity — a passphrase of a few unrelated words is both stronger and easier to remember than a short string of symbols.
Because this account has trading authority over your brokerage, use a password you don't use anywhere else, and consider a password manager.
Common problems
"The provided password does not match your current password" — the current password field is wrong. If you signed up with Google you may never have set one; use the forgot password flow to create one.
Your new password is rejected — it's under 8 characters, or the two new fields don't match. Watch for a trailing space when pasting.
After changing it
Your current session stays signed in. Other devices where you ticked "Remember me" may stay signed in too — if you're changing your password because you're worried about account security, also use Log Out Other Browser Sessions. See Managing your browser sessions.
Adding more protection
Two-factor authentication is the single biggest security improvement you can make to this account. See Setting up two-factor authentication.