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' )
);