1. Create a new folder for the game (for example C:\Games\Eris2).
2. Put Eris2Patcher.exe in that folder and start it.
3. The patcher downloads the whole game (about 2.1 GB). Then press PLAY.
4. At every update just start the patcher again: it downloads only what changed.

If Windows shows "Windows protected your PC", press "More info" and then "Run anyway".