How do I add a credit card payment to an incomplete submission:

There are two ways to complete incomplete payments, both of them are described in our following guide:

https://www.jotform.com/help/272-Paypal-Incomplete-Payments-FAQ

Please note that marking an incomplete payment as a complete submission will not charge your customer. That is explained under the section "Completing submissions without payments" of the above linked guide.

In case that you have received a payment in your Paypal account, but your submission still remains marked as an incomplete submission in JotForm, it means that we never received an IPN (Instant Payment Notification) from Paypal regarding payment being processed. In this case, please follow the section "Completing paid submissions" of the above linked guide.

I would additionally recommend making sure that the IPN setting of your Paypal account is enabled:

https://www.jotform.com/help/276-How-to-Enable-IPN-for-Paypal

Please let us know if you need further assistance.

If someone has an incomplete payment and they give me their credit card information, do I need to submit all of the information again or can I complete the payment with the information they already entered?

Unfortunately, there is no way to update the form with the credit card information. In the incomplete payments you can only view the form information and complete the form. Doing this will not charge the user anything as indicated by my colleague.

You can try contacting PayPal if they can help you update the transaction. If not, the only thing left would be to ask the user to resubmit the form again with his/her CC information.