How do I change the payment integration on my form?

an old form i made and used last year has defaulted to PO for payment, how do I change it to paypal to accept payment?

You will need to delete the current payment integration you have on your form, simply edit your form and click on the payment field, clicking on the trash icon will remove the field from your form. 

Once you have deleted the old payment integration, you could only add PayPal, if you do not refresh the page your previous products will be loaded in the new integration. 

Here is also a guide that will help you integrating your form with PayPal: https://www.jotform.com/help/391-How-to-Integrate-Form-with-Paypal 

I would also recommend you to download your existing submissions before to proceed with the change, this way you avoid to lose data. 

These guides will help you doing so: 

How-to-Download-Form-Submissions-as-Excel-CSV-PDF

How-to-export-all-of-your-data-at-once

I hope this helps.