There are mods available for the current Steam version of Geometry Dash that can "downgrade" your game to feel like older versions.
If you still wish to install version 1.9, here is the legitimate, community-approved method. Geometry Dash 1.9 Download Pc
Once downloaded, move the files to a separate folder and run the executable. Option 3: Android Emulator (Mobile Version) There are mods available for the current Steam