It is not possible to change your username.
You can, however, create another account and transfer all of the sets from your old username into your new account. See our FAQ about copying a Quizlet set for more details. After exporting your sets to your new account, delete your old account by logging into that account and clicking "Delete Account" from My Dashboard.
To change your email address or password, click on Settings in the upper right hand corner of the page.

This takes you to a page where you can change your email address and/or password:
