Theme Update Fails

Hi There,
Wanted to update my theme from 4.0.7 to the newest 4.3 version.
The page works with a child theme. So I went to the child theme – and clicked on themeupdate.
But then it displays: “Theme Updates, No Updates available. You are running the latest version! (4.0.7), Check Manually”
If I click on check manually, I come back to the dashboard.. So I navigated to design – themes and saw that there the theme says that a new version is available. When I click on update looks like its updating but the two arrows signaling that it is updating running circles for ever and ever.
So I assume I have to update the theme manually. How can I do this without damaging the childe theme?
Or is there another easier way to solve that problem?
Thanks
maurice