Q. how do I change to a new a search/topic in eBook Central ? "help" screen doesn't address it!

Just above your initial results (list of titles) is a tab called "Search." This tracks the searches you do, and each time you enter a new search in the box at the top, it will remember. Click on the search tab, and then click the search you want to look at again. If you're looking at an individual book, you can still enter a new search in the box on the left above the ereader controls, or use the Search tab at the very top.