To change the Default text error message after the Submit Form please copy the code below and put it into Unbounce LP JavaScript section (bottom in Editor)
<script type="text/javascript">
var d = window.module.lp.form.data;
/* Please change the main error message TITLE */
jQuery(document).ready(function () {
jQuery('#' + d.errorContainerId).find('.error').text(
"Change This Title:"
);
});
/* Please change the individual error messages */
d.validationMessages = {
"email": {
required: "Oops! Something wrong! Please change ME!!!"
}
};
</script>