Solution:1
With the selector [colspan="6"]
you can get to each and set a new attribute:
$('.shop_table [colspan="6"]').each(function() {
this.setAttribute('colspan', '4');
});
console.log($('table').html());
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<table class="shop_table">
<thead>
<tr><th></th></tr>
</thead>
<tbody>
<tr class="aa"><td></td></tr>
<tr class="bb"><td></td></tr>
<tr class="cc" colspan="6"><td></td></tr>
<tbody>
</table>