How do u sort the column in a multi record block either in ascending or descending order?

I used the following code to do that but didn't help...:-( :-(

begin
if get_block_property('block_name',order) = 'column_name ASC' then
set_block_property('block_name',default_where,'column_name' DESC');
else
set_block_property('block_name',order by, 'column_anmeASC');
end if;
end;


In runtime if the user clicks a button it should sort whole column in the oppsite order in which it is present i.e if the column is present in ascending order then should display it in descending order vice versa.

Can anybody please help me, its urgent.