Reduce DPI

I have tried the steps given by the solution, but dilate seem to do just reduce the DPI. It distort my image, and furthermore, I wish that I could set the number of DPI of that image.

If PSP can't do the job, can anybody recommend any graphics software?

PSP doesn't have any options to adjust the dpi to a certain number.  By reducing the dpi, the image will be distorted.  You are taking a group of pixels that may have many different colors and changing them into one color.  That's just the nature of the beast.

I don't know if PhotoShop will allow you to set the dpi or not.  I don't use it cause it costs way too much.

From what I know, by reducing the DPI, the image will not be distorted, it will only look more "plain" or less color. Just like if you scan a picture using 300 dpi and 600 dpi, the image will look the same, only the color will be richer. From the net, you actually can't see much different from the way it looks, unless you print it out.
Anyway, thanks for your help. Happy New Year.

Reducing the dpi, as going from 600 to 300, will distort the image and make the image look more stair stepped.  Increasing the dpi, as going from 300 to 600, really won't do much.  Because the 600 dpi image is based on the 300 dpi.  It doesn't have additional information to draw form to give the image a sharper look.  However, it will reduce the amount of stair stepped look the image has.

As for scanning, the image generated will have a much sharper look when scanned at 600 dpi than 300 dpi.  The scanned image is getting it's information based on the image in the scanner.