Hey Opinio,
You could either do it via FTP:
http://codex.wordpress.org/FTP_Clients
You would then navigate to /wp-contents/themes/ and delete the folder for this theme. Think of FTP like an extension of your desktop, it's just like opening a folder, deleting, etc.
Another method would be through your hosts control panel, chances are you have cPanel (or similar control panel) on your host and they provide a file manager:
https://documentation.cpanel.net/display/ALD/File+Manager
Once the you remove the copy you have, you can upload a new version. :)
Hope this helps.