Welcome to the community! QuickBooks Online gives you the flexibility to edit your transactions such as purchase orders and resend them via email when needed. I'll be happy to show you how.
Resending your p.o is a pretty straightforward process.
For more on how to manage purchase orders in the program, check out this article: Create purchase orders in QuickBooks Online
I hope this helps! Don't hesitate to stop by if you have more questions.