You must create a backup of all your information if you haven't already, you need to move that backup to the new computer. Install the program on the new computer and load the backup, confirm all your information is there and then uninstall the program from the old computer and register your license on the new one.
In the new computer, you need to make sure you install the version of Express Invoice that is compatible with your license. You will need to reach out to NCH Tech Support to get the correct download link if you do not have that. You can create a support ticket for them here: https://www.nch.com.au/support/index.html
**You must make sure you have a working backup, all information is saved locally in your computer so you cannot uninstall the program from the old computer until you make sure your backup works or you will lose all the information you have.