You will need one of these file converters if the old file is anything before version 8.1.
Version 12 can already open version 8.1 - 12 files.
http://www.graphisoft.com/downloads/fileconverter.html
Download and unzip the converter (no need to install).
Open the old file and save.
Then open in your current version of Archicad.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11