In this case, you can remove the old information. You should be able to just delete the company file and it removes everything associated with it.
If anything is still there after the company file is removed you can just manually remove items, customers etc. If you want o start all over again, do a backup for all this old information, then uninstall the program and install it again. Once the program is running you can enter the new information. the old would be on the backup folder.