How can I change the Horizontal Resolution and Vertical Resolution of image?

filename='b.bmp';
info = imfinfo(filename);
info
if (info.HorzResolution>0&&info.VertResolution>0)
   xres = info.HorzResolution;
   yres = info.VertResolution;
   f=7874/xres;
   B= iir(filename,50);
   filename=sprintf('b.bmp');
   imwrite(B,filename);
else
    display('No DPI');
end

I have written this code to change the Horizontal Resolution and Vertical Resolution of image but after creating new image by this code when I am calling the new image by the code It say No DPI

[Walter Roberson]

I do not recognize iir? What does

which iir

show?