how do I download the paid version of Words for Friends in my computer?

You can download it to an iTunes library, but not run it on a computer.

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Just to be clear, you cannot use iOS apps on your computer.

If you purchased the apps on your iOS device you can transfer them to iTunes on your computer to be backed up

iTunes Store: Transferring purchases from your computer

You can purchase them directly on your computer from the iTunes Store.

Opening iTunes, click iTunes Store on the top right then App Store