1. Login to your Magento admin panel 
  2. Catalog --> Manage attributes
  3. Find and select the attribute 
  4. Ensure "Use in Quick Search" is "Yes", to be searchable, else select "No"
  5. Save attribute and results should be updated in 24 hours, if CRON is enabled. If CRON is not enabled, you will need to complete a Manual Sync. If CRON is enabled but you want these changes to reflect ASAP before your CRON runs, clear Magento caches and also do a manual synch. Kindly ensure manual synch is done during a low traffic time.