If you have the Professional or Server version of Help+Manual you can also edit the real-time index directly with the Index Tool in Project > Tools. Note that this tool is only available in the Professional and Server versions.
This tool enables you to edit keywords in all the topics where they occur simultaneously. You can also add keywords to multiple topics and delete keywords from multiple topics at the same time, in one quick operation.
Select Expanded View at the bottom of the Index Tool window to display links to the topics containing the index keywords. Double-click on the Index Tool title bar to undock or redock it.
IDs, Context Numbers and Keywords (Reference)