I want to find and update my email templates, the ones that accompany POs and Invoices.

I'll guide you where you can update your email message for your PO and invoices, Maggie.

You can go to your Accounts and settings to update the email messages for your POs and invoices. I'll show you how:
 

1. Go to the Gear icon, then select Sales.
2. Under Messages, click the pencil icon.
3. Update the message in the Email message box.
   
4. Click Save and Done.

You can learn how to personalise your forms as you create them. To know more, read this article for your guide: Customise invoices, Estimates, and Sales Receipts in QuickBooks Online.

On top of that, I've also added this article to show you how you can record invoice payments in QuickBooks: Record Invoice Payments In QuickBooks Online.

You can always post here if you have further questions about modifying your invoice email in QuickBooks. I'll be around to help you in any way I can. Take care always and have a good one.