Search for "Undertale," add it to your cart, and checkout.
The most popular choice. It handles installation, updates, and cloud saves automatically. You can find it on the Steam Store . Undertale Download PC Game
Best if you want a "DRM-free" version (no launcher required to play). Buy it at GOG.com . Search for "Undertale," add it to your cart, and checkout
Undertale is a paid game (usually around $9.99). Avoid "free" sites, as they often contain malware. Search for "Undertale