Just grab the latest code here:
https://github.com/PKISharp/win-acme/releases/latest
unpack it into a folder you will keep (I used “E:\Let’s Encrypt”), and run letsencrypt.exe in the folder. The multi-site (SAN) mode works only if IIS is set up in certain ways; but the manual mode for a single site is simple and easy, and it sets up a scheduled task in Windows for the needed automatic updates.
Also recently discovered this, have not tested yet but it can come by Chocolatey, which means automatic updates are easy: