how to change dpi in cs5?

While you create a new file in photoshop cs5, change the dpi resolution as your need.

http://answer.recipester.org/question/10926/

Go to Image then Image size. Or Alt+Ctrl+I. This changes the pixel dimensions but I think it's what you were talking about. You make the image smaller it will be fewer dpi.

In Photoshop CS (and assuming in other versions) you can set the DPI when creating a new document, in the field that says "Resolution".

If you have already created a document and want to change the DPI of it, you can do so by going Image > Image Size, and changing the value of "Resolution".

The web displays graphics at 72 DPI (dots per inch)

Not 'possible'! You cannot increase it quality like that.

Let me try to explain, think 96 dpi as in 96 dots and if those dots is small enough it will create an image (look carefully at Newspaper and you will notice all those picture and word are just very tiny tiny dots). Now what your trying to do is, make those 96 dots into 300 dots! That 3 time the original quality, to do this you have to REDRAW the picture yourself so that it will have 300 dots! You could do some resampling (automatically guess work to increase 96 dots by a little bit) but it will not be any good.

The only way for you to change DPI is, converting 96 to 95 and below, meaning you can remove dots so the picture is smaller but the quality is poorer. What you can do is increase the resolution, think resolution as the paper that you draw your dots on. Again by increase the piece of paper, your dots have to SPREAD out to fill the whole piece of paper, in turn this will make the PICTURE look bad. 

TL:DR - Find a picture that have >300 DPI then you can convert it to 300DPI

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