ALTFF,
Change it to "Update by schedule" and ask your host or server admin to setup magento crons to run every minute. this way your indexes will be automatically updated. You can verify later by checking "Updated" column on the right.
examples of cron jobs you will need on the server:
*/1 * * * * php -c /bin/magento cron:run
*/1 * * * * php -c /update/cron.php
*/1 * * * * php -c /bin/magento setup:cron:run
but better ask your server admin people to do this for you and verify if crons are running without errors. they will need to use correct php instance and permissions and file ownership may have to be adjusted.
hope this helps,
