Alter table with arrays

Hi !

Help me please to alter table with arrays .
The table was created :

create stat (st char(20), stv float[][], str float[][]);

Usualy I add new row as follow:

insert into stat
select '4090903' as st,
'{{0,0,0,0,0,0},{0,0,0,0,0,0}}' as str,
'{{0,0,0,0,0,0},{0,0,0,0,0,0}}' as stv ;

And now I would like to increase the number of colomns of array from
two to tree. What is the way to fulfill these operation?

Thanks .

Igor