Installed theme not working

So I bought a word press theme off theme forest and it gave me the .zip file which is what I needed to upload the theme up on the web, although it states that my upload is incomplete because the stylesheet is missing (its not :S), and now I can’t install it again because it says it already exists…But I can’t get rid of that broken theme, there is no option to delete it, it just says incomplete download…What should I do? Help? I have never used wordpress before