You need something along the lines of

<?php
$nodes = node_load_multiple(array(), array('type' => 'page'));
foreach($nodes as $node) {
  // Only modify nodes that have something in the body field
    $node->field_select['und'][0]['value'] = 'yes';
    node_save($node);
}

Modify as needed. Don't forget to clear the caches afterwards.

You need something along the lines of

<?php
$nodes = node_load_multiple(array(), array('type' => 'page'));
foreach($nodes as $node) {
  // Only modify nodes that have something in the body field
    $node->field_select['und'][0]['value'] = 'yes';
    node_save($node);
}

Modify as needed. Don't forget to clear the caches afterwards.

Another option is installing the Administrative Views and Views Bulk Operations modules, and pointing to Change Value, as seen in the below screenshot