$() returns an array with the resulting objects, since you only have
one you have to pick the first one:
var button = $("#<%=b1.ClientID %>")[0];
You can then change the text using the value property:
button.value = "Yes";
The complete script will then be:
<script type="text/javascript">
$(document).ready(function() {
var button = $('#' + '<%= btnSelectAll.ClientID %>')[0];
button.value = "Testing";
});
</script>
T.
