Since these older builds do not use the modern auto-updating launcher, you must install them manually: April | 2015 - Yandere Simulator Development Blog

Downloading a specific 2015 build of requires using third-party community archives, as the official Yandere Simulator website typically only hosts the most recent launcher and stable demo builds. Because these older versions are prototypes, they are often maintained by fans rather than the developer. Where to Find 2015 Builds

If you are looking for the original 2015 builds of Yandere Simulator

Always scan any downloaded file with an up-to-date antivirus program.

If you are looking for this specific build or more information on the game's evolution, here are the safest ways to navigate it: Finding Historical Builds