System Folder Overload

Today I thought I would download an old game from Steam. It was in my Steam Library. I wanted to see if it would run on my new Macbook Air. The file size was about 60 gig. I had about 70 gig of free space. The download took a long time and stopped after about 95% was downloaded. A Steam message said the game was outdated and the download could not be completed. Well that obviously meant the game wasn't going to run on this computer.

I wanted to reclaim all the space the game had taken up. Checking storage, I found that the System file was holding about 95 gig. So that's where the defective game was sitting. But I can't get at the System files to delete the game. How do I proceed? Thanks.