To access your settings page you must be logged in. On the upper right corner of the site you'll see your profile picture and your name. Mouse over your name to see more options:
Click on the "account settings" link to open your settings page. Then choose the Password link in your settings menu:
You'll receive an email with a special link. Clicking this link will take you to We Heart It, log you in automatically and let you create a new password.
This link will only work for 24 hours, so make sure you check the email and create the new password right away.
On this page you can change your current password. Just fill in your current password and the new one and click the pink "save changes" button.
The next time you log in to the site you'll have to use the new password. Your old password will stop working.