Hi Axel,
First download the theme to your PC from our account.
You can update the theme manually via FTP or just activate this plugin and follow the instructions below:
1. Go to Appearance > Themes.
2. Click on the ADD NEW button.
3. Click on the UPLOAD THEME button.
4. Click BROWSE and choose the zip file that you have downloaded from your account via our site.
5. Click the INSTALL NOW button.
If you have any problems, please let us know.
Thank you!