I assume you're using the free version, otherwise you would see the backup /restore buttons under Tools. So here's the bad news....
When you add an item, a customer, and/or create an invoice, all of these actions (in the free version) are recorded in windows registry. There is no user database holding the information.
Yes you can use the backup facility to backup the data files, but you cannot restore them unless you pay for the program. I think that's a cynical way of getting you to buy but hey...that's life.
So, if you could be bothered, you could export all of the relevant registry settings, then import them back into your registry once you have reinstalled the program, or put it onto a new machine if that's what you're doing.
Personally I stopped using it because if you make a lot of records, your registry is going to bloat even worse than MS software.
You'll find all the data in the registry at HK current user\software\nch software