how to change image resolution but keep the size (dimensions) same?

i have some images having 200 microns per pixel resolution and dimension is 1024x1024 (grayscale image). how can i change it to 400 microns per pixel and keep the dimensions same (1024x1024)?

[Bilal]

images are of pgm format.

Answer 1

[Matt J]

Blur the image with a 2x2 averager perhaps?

convn(YourImage,ones(2)/4,'same');