Linux libpq dbupdates

As a newcomer to postgres/libpq, I would be grateful for some help.
I can see my way clear to effecting most operations using libpq & C
However, I see no fluid way of formulating an update command as there
seem to be no functions to identify the appropriate fields (typically
the index fields) in order to complete the 'WHERE' portion of the
command
string.

Suppose that a number of tuples are returned from a query, which may
stimulate
one or more items to be updated.

Does anyone have a good example of achieving such a relatively simple
and
common function?
OR
Is there some good comprehensive documentation (with examples) re libpq
usage.

Any pointers welcome

Many Thanks

Tom Knowles