change field type and length

I've created table with postgresql, and so far so good, but I want to change
Field type and field length, I've tried to find command line and
documentation postgresql to change table field, but I can't get it. so there
are any trip to change that field...?

CREATE TABLE "dat_jab" (
   "code_jab" numeric(2,0) NOT NULL,
   "nm_jab" varchar(25),
   CONSTRAINT "dat_jab_pkey" PRIMARY KEY ("code_jab")
);
CREATE  INDEX "dat_jab_code_jab_key" ON "dat_jab" ("code_jab");

***  want to change Field Type "code_jab" numeric(4.0) , and field name
"nm_jab" -->"nm_cdjab"


regards

--dv