Changing primary keys

Hi,

The table 'house' was created with the below code (partial)

CREATE TABLE house (
   house_id bigserial unique not null,
   phone bigint not null,
   address varchar(75) not null,
   primary key(phone, address)
);

My question is how can I switch the primary key from (phone, address) to 
house_id, now that the table is created and records have been inserted.

Thanks,
C