how to change the dpi of an image

Use Paint.NET

http://www.dotpdn.com/downloads/pdn.html

Go to Image > Resize > Change its resolution (dpi)

As I said the resolution in the Paint.NET is the DPI (pixel/inch)

I suggest using GIMP; it is a free image-editing solution, and it allows you to change the resolution, in pixels per inch, in both the x and y dimensions. I believe that this program can accomplish what you are looking for.

And I would take this one free tool: http://www.irfanview.com

To change DPI for the currently loaded image, click the "i" icon ("i" for information) on the IrfanView toolbar. Change the DPI, click OK, and save the image.