Is it possible to override the contact_plain.php which is in the folder: ../plugins/system/yjsg/includes/html/com_contact/contact with putting it to the ../templates/eximium/html/com_contact/contact .
Are you sure your are posting in product assigned forum?
What is your CMS version?
: Joomla 3.x
Link to installation
Have you edited any files? If yes tell us which ones.
Is this personal or client website?