import pyfits
from pyfits import open as pyopen

FitsName = 'Whatever.fits'

FitsFile = pyopen(FitsName)

image= FitsFile[0].data
header = FitsFile[0].header

header.update('RESTFREQ', 1.420405716410E+09)

NewFitsFile = pyfits.core.writeto(FitsName,image,header=header,clobber='true')