1 - The company already exists
Enter the company and go to Restore data

Indicate where the backup is located :

No file name to indicate since it will only restore a file of type company_code.zip of the code of the company where we are. (In the example here, demo21.zip)
If the backup was made with a password, it asks to type it:

2 - A backup from a new company (not existing in the program)

From the main menu, go to "Tools", "Create a company from a zip file".

Select the zipfile from where it has been saved.