Unzip the istore.zip theme file. To upload the theme via FTP, you will need to use FTP client software on your computer.

You can use a free FTP client program called FileZilla. Learn how to connect to FTP server using FileZilla here.

Once connected to your FTP server, browse the remote server path to /[Wordpress-Root]/wp-content/themes/ and upload the istore theme folder.
Once the upload is completed, go to WordPress Admin > Appearance and you will see the iStore theme there. Click activate theme button.