Hello @skullbag,
Thanks for asking. 
As you have already installed child theme, all you need to do is copy portfolio.php file from /wp-content/themes/x/framework/functions/global/ and paste under /wp-content/themes/x-child/framework/functions/global/. You may need to create additional folders.
Next you need to focus on line number 241 that starts with <a href="<?php echo $project_link; ?> and in that line replace target="_blank" with target="_self". This will make portfolio button link to open in same page.
Please note that custom development falls outside the scope of support we offer but I have tried to point you in right direction believing that it will be your responsibility to take from here on.
Thanks.