If your credit card is reaching its expiration date, PayPal will contact you and ask you to update it.

It is very easy to change your debit or credit card details on PayPal. Simply log in, click "Profile" at the top of the page, click "Credit/Debit Cards" under "Financial Information," select the card and click "Edit," and then enter your new information and click "Save." If you attempt to make a payment through PayPal with an expired card, it will be denied.

The PayPal help center can be found at https://www.paypal.com/us/cgi-bin/helpweb?cmd=_help.

See also, Who can I contact if I have problems paying with PayPal?