Next and Previous Button class change

Hello, i was able to change the submit button class by adding this code to my theme's function file

add_filter("gform_submit_button", "form_submit_button", 10, 2);

// filter the Gravity Forms button type
add_filter("gform_submit_button", "form_submit_button", 10, 2);
function form_submit_button($button, $form){
    return "<button class='fancy_button' id='form_submit_button_{$form["id"]}'><span>Continue to Payment</span></button>";

but when i tried to do the same for the Next and Previous buttons, [Multi Page buttons ] it didnt work,
here is the code i used,

add_filter("gform_next_button", "form_next_button", 10, 2);
add_filter("gform_previous_button", "form_previous_button", 10, 2);

// filter the Gravity Forms button type
add_filter("gform_next_button", "gform_next_button", 10, 2);
function form_next_button($button, $form){
    return "<button class='fancy_button' id='form_next_button_{$form["id"]}'><span>Next</span></button>";
}
// filter the Gravity Forms button type
add_filter("gform_previous_button", "gform_previous_button", 10, 2);
function form_previous_button($button, $form){
    return "<button class='fancy_button' id='form_previous_button_{$form["id"]}'><span>Previous</span></button>";

Help me :)

could u give me more information on how to use jQuery to append/change the class name for now.
like where to start

Kevin,
im kinda noob with codes stuff,
could u tell me where to insert the code you just pasted please?
regards

Hey Kevin,
i insterted this into my header.php file

<script type="text/javascript">
jQuery.noConflict();
jQuery(document).ready(function($) {

   $(".gform_previous_button").addClass("fancy_button");
   $(".gform_next_button").addClass("fancy_button");

});
</script>

it worked on the first page, but when u press next, the next 2 buttons is still the same :/
take a look here
https://psdtowebsite.net/order
im confused