I appreciate you for getting back to this thread, @bjk20080. Let me chime in and provide an update about updating payment instructions in QuickBooks. 

As mentioned by my colleague above, the option to edit payment instructions is currently unavailable. As a workaround, you can create your own instruction and add it in the Memo field.

Here's how:

1. Sign in to your QBO account.
2. Go to the Sales menu and choose the All Sales tab.
3. Select to open the payment. 
4. Enter your payment instruction in the Memo field. 
5. Once done, click Save and Close.
   
   

Otherwise, you can follow the steps provided by my colleagues above on how to customize your invoice before sending it to your customer.

I'm also attaching this link to help you put payments into the Undeposited Funds account before you combine them into a deposit.

Keep in touch if you have other questions about managing your invoices in QuickBooks. I'll be around to assist you always. Stay safe and take care.