use photoshop to change image resolution for iphone4 vs iphone3

I have a psd which is for i开发者_C百科phone4 640x960. Inside photoshop, the image size says the resolution is 72pixels/inch. How do I decrease the resolution by half for the iphone3's 320x480? I tried changing 72 to 36, but that didn't do anything.

This isn't really a programming question, but the DPI is just a parameter on bitmapped images, given that they don't otherwise have any concept of physical size. You can set it to anything without affecting the actual pixels.

You want to adjust 'Image Size', which when I last used Photoshop (a very long time ago) was under the 'Image' menu. You then want to set the size in pixels to 320x480.