Normally pressing the ‘Re – Index’ button at the default search settings page above (..admin/config/search/settings) would suffice but if the nodes are in the thousands you will need to use Drush instead.
Logged in using ssh, use the drush command
And it will result in this:
It can take up to 4 seconds per 100 indexed nodes so this may take time. Checking admin/config/search/settings again afterwards will show 100% indexed nodes: