The email that is send is based on the Membership - Changed password template. You can edit this under the email templates application.
https://docs.kentico.com/k12sp/managing-users/user-management
Generating a new password
You can generate a completely new password by clicking the Generate new password button. The affected user receives an email, based on the Membership - Changed password email template, containing the generated password and a recommendation to immediately change it. This password complies with the set password policy (a string of 8 characters containing at least one non-alphanumeric character by default).