You can install this theme by uploading the theme zip file in WordPress admin panel.
Go to Appearance > Themes > Add New > Upload Theme). You won’t lose any of your content (posts, pages etc.) when you activate it.

Installing Xmas Lite Pro

Go to Appearance > Themes > Add New
From top, Click on Upload Theme
Choose the ZIP file you’ve downloaded & Click Install Now
Click on Activate to activate the theme

Activating your license

Go to your website & log in
Go to Appearance > Theme License
Paste your theme license key into the License Key field
Click on Save Changes

Info: Without activating license, you will not receive any updates of Xmas Lite Pro