You will need to look up the specific AppID for Geometry Dash (322170), along with the DepotID and ManifestID corresponding to the late 2014 / early 2015 version of the game.
While 1.9 doesn't have a built-in FPS bypass, you can use external tools to achieve higher frame rates, which makes the wave mode much smoother.
Ensure the 1.9 files are stored separately from the current version to prevent save file corruption.
To help find the right version, what specific featureLet me know:
Double-click the GeometryDash.exe file inside your extracted folder to start playing.
Some creators have developed specialized editors that allow you to play and build in the 1.9 engine.