Backup procedure depends on which device you are using Book Keeper.
For Book Keeper Windows:
By default Book Keeper database file is saved on Desktop you can copy that file in any external drive or you can attach it on mail.
If you don't know how to email company file on Computer, please check this video:
For Book Keeper iOS:
Exit your Company. You will see list of companies > Left swipe on company name > More > Backup on email.
For more details you can refer to below video link
For Book Keeper Android:
Exit your Company. You will see list of companies > Click three dots against company name > Backup company > Email/SD card.