Please note that this is not a theme issue but rather it is with the setup of your hosting as installing a theme should be straightforward when you do it from the Wordpress admin.
But since you are getting the
Destination folder already exists you might have tried to install X but the installation didn't go through successfully. Possibly because of the file permissions on your server.
You have two options, either you could get in touch with your hosting provider and have them check your site's permission. You can find more details about it here:
Also, ask them to check the directory
wp-content/themes and have them clear it out so you can have a fresh install of the theme.
If you are still not able to get it working, kindly provide us with the link to the site in question with the Wordpress admin and FTP details. You can get your FTP details from your hosting provider. Please post the details in a Secure Note.