Hi,

<function name="write" model="ir.model.data">
   <function name="search" model="ir.model.data">
       <value
           eval="[('module', '=', 'sale'), ('name', '=', 'mail_notification_paynow_online')]" />
   </function>
   <value eval="{'noupdate': False}" />
</function>

After setting the noupdate=false, disable the mail_notification_paynow_online template.
Use the code:-

<record id="sale.mail_notification_paynow_online" model="ir.ui.view">
   <field name="active">False</field>
</record>

And now you can reset the noupdate property of mail_notification_paynow_online template.

<function name="write" model="ir.model.data">
    <function name="search" model="ir.model.data">
        <value
            eval="[('module', '=', 'sale'), ('name', '=', 'mail_notification_paynow_online')]" />
    </function>
    <value eval="{'noupdate': True}" />
</function>

After performing all the above steps, we need to replace the div section of the button in  mail_notification_paynow template.

<template id="mail_notification_paynow" inherit_id="mail.mail_notification_paynow">
 <xpath expr="//tr[2]//div[1]" position="replace"></xpath>
</template>

Hope it helps