Hej Helena,
Jeg fandt en guide på engelsk, hvis du har brug for hjælp til at oversætte den, så må du sige til, men prøv at følge den her og se om det kan løse dit problem.
Download Media Creation Tool from Microsoft:
https://www.microsoft.com/software-download/windows10run it and select "Upgrade this computer" (or you can select "create installable media" (DVD / USB) if you want to make clean installation, but it wasn't my case) - download will start immediately
if you get stuck during installation (after downloading) on "checking for updates" message, then open Command prompt as administrator (how to do it) - while installation window 10 is still open and running - and type command: net stop wuauserv. This stops windows update service. After a minute you should see "Checking computer" instead of "checking for updates" in Windows install window and installation goes on...
if you get stuck again during next "checking for updates" process, repeat the last step - stop windows update with net stop wuauserv. I had to do it two or three times during "checking for updates".