I found a way to make this work, although not ideal, and thought I would share, in case anyone is looking for the same thing before there is a better way:
Go to your theme’s twig file directory and modify buttons.twig. I’m using vortex. All I did was add an id to the span tag for the label and then using javascript swapped the inner label text with what I want it to say.
<span id="ticketsButton" class="ai1ec-hidden-xs">
<script>
document.getElementById('ticketsButton').innerHTML = "Register";
</script>
You also need to make a change to ticket.php under /app/view/event/ticket.php to change the label on the individual event page.
