If you still have access to your Ulule account, click your name or profile picture at the top right, then Settings.
On this page, open the Password tab, then enter your current password and the new one.
Note that this change logs you out automatically, so you have to log in again with your new password.
If you can't access your account and forgot your password, you can reset it here: https://www.ulule.com/password/reset/
This link is also available on the login page, under “Forgot your password?”:
Make sure you enter the exact email address registered in your Ulule account.