If you’ve forgotten the password and are unable to log in to WHM, there is no specific function for resetting the password for WHM. Instead, use the method described in this guide to reset the password for cPanel (since the same credentials are used to access WHM as those used for cPanel). Use the steps described under “Via cPanel” in those instructions.
Keep in mind that you need to do the password reset for the cPanel account that owns all accounts you have access to on the server.