There are many live scenarios in sitecore application where we need to update the index to reflect the changes on website (UI). As sitecore recommended we use solr/lucence search index to play with huge data or search.
Some time we need to update some items or particular one item to update the index file.
So here is the way to update the single item in index file:
1. Go to sitecore
2. Go to developer tab: by default its hidden to show a developer tab, right-click a tab, and then select the tab to show > developer.
3. Or go to ribbons > my toolbar >Customize>Add Re-index Tree under indexing tools
4. Now select the item which you want to rebuild the index
5. Click on Re-index tree button > it will re build the index of selected item and its descendants.