If you changed to HTML you can add the text attribute to the button definition in the JS:
(as posted here: https://answers.splunk.com/answers/147151/change-the-submit-button-text-in-splunk-6-1-simple-xml-dashboard.html)
var submit = new SubmitButton({
id: 'submit',
el: $('#search_btn'),
text: 'Update'
}, {tokens: true}).render();
If you want to keep it as SimpleXML you can add the following to your page JS file (within the document ready function):
$('.submit').html('Update'); // during loading
$('.submit').bind("DOMSubtreeModified",function() { // when SplunkJS tries to change it
$('.submit').html('Update');
});
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