How can an affiliate change their password?

Affiliate accounts are tied directly to WordPress user accounts, meaning, you cannot be an affiliate without also having a WordPress user account on the website where AffiliateWP is installed and active. 

If an affiliate has forgotten their password, they can click the Lost your password? link as seen on the affiliate login form:

The affiliate password can also be reset on the user account through the WordPress admin by following the steps in this article: Resetting Your Password in WordPress.