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