To download the Jupiter X theme:
1. Go to ThemeForest and log in to your account.
2. In the upper right corner, hover over your username and click on Downloads.
3. Scroll down to Jupiter X and click on Download All files & documentation.
Please note: When you use Download all files & documentation in the downloaded zip file you will find 2 theme folders, Jupiter V6 and Jupiter X. if you want to download only Jupiter X download only installable files from the same screen. Along with the theme files in the folder, you will find Child theme, Plugins and the Documentation.
For more details check this link.
4. Extract the downloaded .zip file into a folder. These files will be needed for the theme installation.