Hi,

You can try to extend the existing records for the email template through codes or you can try it with the UI

<record id="auth_signup.reset_password_email" model="email.template">
        <field name="name">Reset Password Custom Name</field>
    </record>
Here I have changed the value of the name field only. You can also change any value as per your wish.

for changing the body of the templates

<record id="auth_signup.reset_password_email" model="email.template">

    <field name="body_html" type="html">

        <div>// Add you content</div>

    </field>

</record>

Hope it helps