I want to use my own data to use for testing, can we put this in over top of the Demonstration Company?
No, the Demonstration Company has to remain.
But, you can create a NEW company (Admin / Other / Create a new Company), and restore your own backup into that. Because the licence number is carried in the backup, that new copy of your data will work.
We would recommend that if you do this you assign a logo to that copy / play set of data so that it is very clear you are not working in your live system.
Instructions for a Test Data System
To read about assigning a logo to your copy set of data read Adding Logos to Print on Invoices