Changing 'Primary Key' in Oracle Table

Changing 'Primary Key' is two steps process as under:

Step 1:DROP existing primary key:

alter table my_table drop constraint my_pk;

or,

An alternative syntax to drop the existing primary key (e.g. if you don't know the constraint name):
alter table my_table drop primary key;

Step 2: Re-create the primary key:

alter table my_table add constraint my_pk primary key (city_id, buildtime, time);

However, if there are other tables with foreign keys that reference this primary key, then you will need to drop those first, do the above, and then re-create the foreign keys with the new column list.