geotiff: using libtiff "tiffset" to alter tag

Hello,

I am trying to use the libtiff command "tiffset" to alter a geotiff
tag. The tag in question is:
Tag 33550: -1237.520056,1237.520056,0.000000
or:

I have tried using the tiffset command using the count option, but I
keep
getting a usage error no matter what I try. Could someone post an
example of
how I would use tiffset to change this tag to, for example:
1237.520056,1237.520056,0.000000

Thanks,
Brendan