I would recommend making the buttons toggle between ASC and DESC for the field to be ordered on. For example:

WHEN_BUTTON_PRESSED Trigger for name button:

begin
if get_block_property('BLOCK_NAME',default_where) = 'NAME ASC' then
set_block_property('BLOCK_NAME', default_where,'NAME_DESC');
else
set_block_property('BLOCK_NAME',default_where, 'NAME ASC');
end;