How to ALTER TABLE and SET DEFAULT in CUBRID?

I have the following table already created in my database.

CREATE TABLE "tbl_users"(
     "id" integer AUTO_INCREMENT,
     "email" character varying(100) NOT NULL UNIQUE,
     "join_date" integer NOT NULL,
     CONSTRAINT pk_tbl_users_id PRIMARY KEY("id")
);

I want to alter the table schema and set the default value for join_date column to be the current timestamp in UNIX time. I tried the following SQL, but I keep getting "Syntax: syntax error, unexpected IdName" error.

ALTER  tbl_users ALTER  join_date SET DEFAULT UNIX_TIMESTAMP(CURRENT_TIMESTAMP());

How should I do?