1. Browse Free and Premium Themes within Shopify
Navigate to Admin > Themes
Click on Visit the Theme Store on the top right
After browsing and choosing a theme, click Get Theme
When prompted, log into your Shopify store.
2. Upload Your Purchased Theme
If you've purchased a theme outside of Shopify, you'll be provided a zip file, which you'll need to upload to install the theme. It's quite a simple process.
In Admin > Themes > Upload a theme
Choose the zip file and click upload.
Once you've installed the theme, make sure you activate it for it to show up on your store.