How to change wordpress registration form submit button value?

you would have many options to change button text like jquery and if you know about that registration form from where it is coming from.

By using jquery you would have to add below code at the end of the footer.php file like as below:

<script>
  $(document).ready(function(){
    // Change text of input button
    $(".ur-submit-button").prop("value", "Input New Text");
   // Change text of button element
   $(".ur-submit-button").html("Button New Text");
<script>