the best way to update the template user is to delete the template user using xml and create the same template user with your modification in it.

you can delete the user using 

<delete id="auth_signup.default_template_user" model="res.users"/>

and create it again(make sure you use the proper ID)

       <record id="auth_signup.default_template_user" model="res.users">
            <field name="name">New Template User</field>
            <field name="login">portaltemplate</field>
            <field name="active" eval="False"/>
            <field name="groups_id" eval="[(6, 0,  [ref('base.group_portal')])]"/>
        </record>

Hope this helps!