Copy over and replace all files in the "SILVATALE" folder into your UNDERTALE directory. SILVATALE is built on the most recent Windows patch of UNDERTALE on Steam (V1.08), it is not intended for any other version of the game, including MacOS and Linux releases. Compatibility is not guaranteed.
Default windows directory/path:
C:\Program Files (x86)\Steam\steamapps\common\Undertale
Using a program to patch xdelta files (Delta Patcher recommended!!) replace the current version of your data.win.
It is also recommended to lock your games framerate to 30 FPS (using a tool/program like NVIDIA Control Panel) due to an issue regarding monitor refresh rates not divisible by 30 (such as a refresh rate of 165Hz). This can cause noticeable skips or pauses in audio and/or cutscenes if not fixed manually.
Enjoy!