There are several ways to install WordPress theme, we recommend using FTP but you can choose other ways. Please read more about WordPress theme installation on the official website: http://codex.wordpress.org/Using_Themes

Upload via FTP:

Step 1 – Please download the Oswald Lite version here and unzip it;
Step 2 – Upload “oswald” theme folder to the theme directory (wp-content/themes/) of your WordPress via FTP;
Step 3 – Log into your WordPress dashboard and activate “oswald” theme in Appearance -> Themes section;
Step 4 – The theme is ready. Please go to Appearance -> Customize to modify the theme to your needs.