Open "Terms & Conditions" link in a new tab on checkout page

I am assuming you are using the default template and English, otherwise the instructions may be different, but this should get you started.

In catalog/view/theme/default/template/checkout/payment_method.tpl, you will see a block of text like this:

<div class="pull-right"><?php echo $text_agree; ?>

This is what prints the link. This variable $text_agree is set in catalog/controller/checkout/payment_method.php. Here's the logic that does it:

$data['text_agree'] = sprintf($this->language->get('text_agree'), $this->url->link('information/information/agree', 'information_id=' . $this->config->get('config_checkout_id'), true), $information_info['title'], $information_info['title']);

You want to change this to

$data['text_agree'] = sprintf($this->language->get('text_agree'), $this->url->link('information/information', 'information_id=5' , true), $information_info['title'], $information_info['title']);

then last you have the definition of the link. In catalog/language/en-gb/checkout/checkout.php change

$_['text_agree'] = 'I have read and agree to the <a href="%s" class="agree"><b>%s</b></a>';

to

$_['text_agree'] = 'I have read and agree to the <a href="%s" target="_blank"><b>%s</b></a>';