Thank you for contacting the PayPal Community Forum and welcome as a new member!
If you are unable to edit the card details in order to update the expiry date of the card please try and remove the card first. Once you have removed the card you can add the card again using the new expiry date.
If you have a pending payment/ billing agreement that is due to be paid using this card it may prevent you from updating any details on the account. This payment will need to show as completed before the card details can be changed.