Only by saving to 19 and then opening in 19 and saving to 18.
You can only save back one version at a time.
You might be able to download 19 from the 'MyArchicad' link at the top of this forum if you don't have it.
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