If you are currently a free user and would like to upgrade to Premium version from free theme then its quite easy. Here is the simple steps to follow to upgrade your theme from free to pro.
Step 1: Download the theme
Once you purchase the premium version an email will be sent to you with the theme file ( .zip extension ) as attachment. Download the premium theme from there.
Step 2: Upload and activate the theme.
On your admin dashboard, Go to Appearance->Themes click on the "Add New" button at the top. On the next screen, click on the Upload Theme button at the top. You will be prompted to choose the zip file that you downloaded earlier. Select the file and click Install Now. Once your theme is installed, you will see a success message along with the link to activate and preview the theme. Click on the activate button, and you’ve successfully installed and activated your Pro theme.
*( Quick Security Tip: Remove the plugins/themes from your directory that you are not using. )