GIMP reads PSDs to some extent. Surely all flashy features will be left out or misuderstood, but bitmap data from the layers transfer to GIMP's layers. Try it, it's freeware.
Also ON1 Perfect Layers and Serif Affinity Photo read PSDs. They have functional limited time demos available.
Installing old version of software is a risk if you have a new one already in use. At least be sure that you have a proper system backup available in case that your new version gets sick. Preferably use an old computer for old software.
Adobe has in Photoshop had a long term compatibility option in the Save as -dialog. I do not know how those files are saved. Another answer suggests to try at first with Photoshop. Of course this is the best option for first try assuming it's not already proven to be impossible or useless.