How to change invoice reminder mail body on each and every reminder? it's not showing in receiver mail

Hello there, prageeth.

You can set a default email body reminder to use across all invoices. Let me show you how:

1. Go to the Gear icon and select Account and settings.
2. From the left menu, click Sales.
3. Under the Reminders section, set the email information.
4. Click Save and then Done. 

If you wish to set different email body reminder to each invoice, you can customise a template and use it to every reminder. You can check out this article for more details: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Let me know if you have other questions. You take care and have a great day!