Hey Christian,
Thanks for the screenshot. If you want to change the contact admin link, you have to edit the following file.
JoomlaFolder/components/com_payplans/templates/default/payment/default_complete_cancel.php
You would have to replace the 2 lines here(
http://take.ms/a0RGY) with something like below.
<a class="btn btn-large" href="https://yourlink.com">
<?php echo XiText::_('COM_PAYPLANS_ELEMENT_EMAIL')?>
</a>