Grammar issues and hacking the plugin

I've found a few issues with the default grammar. If I want to change it where would it be recommended I do so. For example in payment.form.php

<?php echo __('You currently have a subscription for the ', 'membership') . $sub->sub_name() . __(' subscription. If you wish to sign up a different subscription then you can do below.','membership'); ?>

I see this is language enabled but where would I change that phrasing? If I hack this file will it revert on updates?

Thanks in advance.