In order to install a new VirtueMart theme you should upload its folder including all the files in thecomponents/com_virtuemart/themes directory.
You can check our FTP tutorial for more details on how to upload files:
http://hostlantern.com/clients/knowledgebase.php
Then you should navigate to your Joomla administrator area-> Components-> VirtueMart-> Admin->Configuration->Site and pick the new theme from the Select the theme for your Shop drop down menu.
Save the changes.
Also, you can click on the Configuration link besides the drop down menu mentioned above and tune your theme.
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