SQL query to change the value of a Custom Field

If you’re looking for raw SQL query, then this should help:

UPDATE <PREFIX>postmeta SET meta_value="temp4" WHERE meta_value="temp1" AND meta_key = 'post_template'

If you want to run this query from WP, then you can use this:

global $wpdb;
$wpdb->update(
    "{$wpdb->prefix}postmeta",
    array( 'meta_value' => 'temp4' ),
    array( 'meta_key' => 'post_template', 'meta_value' => 'temp1' )
);