If the document was sent, there is no way to delete or change email of one of the recipients who had fields assigned to them (aka Signer).
You can delete a CC recipient on a sent document by going to Recipients, clicking the recipient's name > Delete this recipient.
If you have already updated the email address of this customer in the PandaDoc Contacts, you will need to make the document duplicate, delete the recipient from the duplicate and add them once again. As the result, this recipient will be added with the updated email address. Then please send this duplicate and delete the original document.