There are two ways that an affiliate can have their password reset.
Affiliate resets their own password
An affiliate can go to the Tapfiliate sub-domain of the program and reset their password by clicking on the "Sign in" button at the top > Forgot password link.
If the email they submit matches one in the program, the rest link will be sent.
Advertiser resets the password for the affiliate
An Advertiser can launch a message containing a temporary password. This can be done by clicking on the affiliate. In the side-panel, the user can click on the three dots at the top. The option to reset the password is there: