Go to your dashboard and select Appearance (on the left-hand menu bar) and then Editor. From there, you can view your Templates, followed by php files on the very right. Select the “footer.php” from the php files, scroll down to the closing body tag, and place the retargeting pixel directly above it. (Note: if you don’t see “footer.php” then just hit Control or Command F on your keyboard, type in “footer.php” and hit Enter or Return.)
If this sounds too complicated, consider finding someone online who can help you for an hourly or task-based rate.