create table ttt(col_a decimal(9,4))
insert into ttt( col_a )  values (1.2345)
insert into ttt( col_a )  values (2.3456)
insert into ttt( col_a )  values (3.4567)
--- Check before and after 
select  col_a Before , ROUND(col_a, 3, 1) After from ttt
 
-- Update Data 
update col_a set ttt = ROUND(col_a, 3, 1)
HTH 
