FW:How to modify a tuple returned by SPI_execute

Sorry for offtopic, since the pgsql-general ML does not seem to work I post
here.

I checked the documentation, and still do not get it. I can use SPI_copytuple
to return a modified version of a tuple, but how do I modify a column of
type HeapTuple.

In my case, I just want to modify a INT32 column for sorting.

Any ideas?

Regards,
        Mario Weilguni