from astropy.io import fits

hdulist =fits.open('F140W_TESTsubpix_sc0642_fr1_drz_sci.fits')

prihdr = hdulist[0].header
scidata = hdulist[0].data

#print(scidata.shape,scidata.dtype.name)

exptime = prihdr['texptime']
gain = 2.5

scidata *= exptime/gain
prihdr['GAIN'] = gain, 'e-/ADU'
prihdr['BUNIT'] = 'COUNTS'
hdulist.writeto('F140W_TESTsubpix_sc0642_fr1_drz_sci_countsg.fits')