Hi,

Multi row variable set is accessible as JSON string. You can create onchange catalog client script and update the field. Refer the sample code

    var mrvs = JSON.parse(g_form.getValue('mrvs_variable_set')); 
    // Replace mrvs_variable_set with actual variable set name

    for (var i = 0; i < mrvs.length; i++) {
        mrvs[i].hide=false; // replace hide with actual variable
    }

    g_form.getValue('mrvs_variable_set',mrvs);
    // Replace mrvs_variable_set with actual variable set name

Even in backend you multi row variable set is store as JSON. You can follow similar script to update the mrvs_variable_set