This post will help you with walk through on how to Re-index the list in SP Online.
So lets get started :)
1. Go to your list and click on List tab in SharePoint site.
2. Now,click on List settings on the ribbon.
4. Now scroll down and click on Re-index list
5. It will ask for confirmation and click re-index
6. Once done, now click OK
Your list will be re-index when next time search crawl runs.
Hope it helps
Happy SharePointing :)