1. For a normal update of KOReader, using windows, you just open two windows explorer windows. One with the /adds folder, on your device, displayed and one with the download open in your compression program (like WinRAR) showing the "koreader" folder. Then you can just drag the "koreader" folder over to the adds window and drop it there. (You will be asked if you want to merge the files, which you do, then you will be asked how to handle the files. You may have to skip the history and screenshot folders.)
2. Tshering has addressed what is needed in his installation instructions in the first post of the
"Kobo Start Menu 8" thread. Pay attention to the note that mentions your device.
Luck;
Ken