Here is a guide how to install a Shopify theme:
Login to your Shopify online store admin panel.
Navigate to Left Menu -> Online Stores -> Themes
Click on ‘Upload Theme’ button:
From newly opened ‘Upload theme’ dialog, click on ‘Browse’ button and select your theme zip file:
Then click on ‘Upload file’ button
For new theme in list, click on Actions, and then ‘Publish’
From the preview theme dialog click on ‘Publish’ button to confirm:
That’s all. Then open your site storefront and view your new theme display.