Hii, here is a quick tutorial on how to change versions. This is for anyone who wants to rollback to the 2015 update (2.0) or Legacy (1.5):

Please note that changing versions will always necessitate a complete reinstall. As such, you may wish to keep versions you will be frequently playing by using Steam's built-in backup function, which can be found under "Installed Files" in the game's properties window.

1. Open Slender The Arrival's properties window, either by right-clicking STA in your Steam library and clicking the "Properties..." option, or by clicking the gear icon when in STA's game page, then clicking "Properties..." there.
2. Click on "Betas", then select your version from the "Beta Participation" dropdown. 2015 is the version commonly referred to as 2.0, whereas 2013 is commonly referred to as Legacy. Click on None to revert back to Anniversary.
3. Wait for Steam to download the new version, launch, and enjoy!