Manual download
Find file here:
https://pgaskin.net/KoboStuff/kobofirmware.html
Also has older versions to allow going back.
1) Download and unpack zip
2) Connect Kobo, but make sure at least 50% charged.
3) Make sure hidden folders are shown.
4) copy CONTENTS of folder / directory created in 1 to .kobo folder/directory on Kobo
5) Make sure it's done and software eject Kobo. Wait for it to react before unpluging cable. At one stage screen may be blank for a few minutes. It varies.
6) Optionally power off/on, but usually not needed.