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You don't. That's what everyone who's been griping about not being able to delete photos from the Photo Stream are talking about.

Once the photos are in the folder on your PC, it's just like any other folder on there containing files. You just pick and choose which ones you want to keep or get rid of. The ones you want to keep, just move to another photo folder.

But the ones on your phone stay, unless you turn off Photo Stream in Settings. If you turn it off, they all get deleted and the PS folder in your Photos App is removed. Turn it back on, and every picture you take from that point forward goes into the stream again.

Think of it as a way to save every picture you take so you can go through them on your PC later and find the good ones.
 
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