Open you broker web login area and there you can see all your accounts you can change your all accounts password there.
If your broker don't allows to change password there then you need to request your broker with sending an email like email@example.com firstname.lastname@example.org first you ask from live chat exact email for password change.
simply don't take any risk and have your back up system's ready.
But your broker have a support system then can change your password and generally they will send your new password to your email address.
This is their support Please contact the customer support team on +44 (0)20 7456 7681 or email@example.com if you have any problems.
Read their FAQ
How do I change my password?
If you wish to change your password, please call us and we will conduct a security check to confirm your identity before amending your account details.
I've forgotten my username/password:
If you have forgotten your username or password, please call us or if you are unable to do so, please email us. We will send you some information that will remind you of your username or password. We are unable to send passwords via email for security reasons.