You don't delete the topic, you remove it from the plan, so it will no longer be included when you print and browse. In the more recent versions it remains available to you in case you change your mind.

The details on how to modify the outline are in your manual, in the programs help files, or within the Customizing the Plan Outline article and they do depend on which version you are using.

The important point is that of course you can change the outline, add topics, and delete (or remove) them.