Editing FITS header info in a macro

[ppp]

Hi all,

I'm having difficulties producing a macro to adjust some values in the header of a FITS file, an example file can be seen here.

I would like to adjust the CRPIX1 and CRPIX2 values to 837.3 and 113.5 respectively, in order to correct some coordinate system associated with the file.

Does anybody have an idea? Thank you in advance,

Peter