To change your password, log into https://account.riseup.net/ and
select the "Passwords" menu on the left. There you can enter your
current password, your new password twice and click on "Change
password".
PS: If you lose your password we
will be unable to help you. Because of that, you might want to generate a
(new) recovery code and keep it in a safe place, since that is a way to
gain access to your account if you lose your password. See also How do I access my account if I forgot my password?