You can change the DPI with the Resample method of MagickImage. The parameters are now described as width/height but it should be resolutionX and resolutionY. This will be changed in the next release.
Changing the DPI from 300 to 600 will resize the image to a bigger size. This will result in an image that has a lower quality so I am not sure why you would want that?
Changing the DPI from 300 to 600 will resize the image to a bigger size. This will result in an image that has a lower quality so I am not sure why you would want that?