I have created a composite primary key (a,b,c) in the table. It has references in other table also.

Now I want to modify the primary key order - (b,c,a).  How to go about it?

You can try this sequence:

1) disable primary key related foreign constraint on child table;
2) disable primary key constaint on present table
3) drop index
4) recreate index
5) enable all disabled constraint

The enclosed scrip will do it:

CODE
FILE: recreate_pk_ddl.sql

set pagesize 999 heading off verify off termout off
set feedback off sqlcase upper newpage 3 rowsize 400
SELECT constraint_name column1,5 column2, 0 column3,
      'alter  table '||table_name||' add constraint '||table_name||'_pk primary key ('
FROM user_constraints where
constraint_type = 'P'
UNION
SELECT constraint_name,10,position,
      decode(position,1,' ',',')||column_name
from user_cons_columns
where constraint_name in (select constraint_name from user_constraints
where constraint_type = 'P')
UNION
SELECT constraint_name, 20,99,'   )' from user_constraints
union
select constraint_name, 30,99,'   using index' from user_constraints
union
select constraint_name, 40,99,'   pctfree  10' from user_constraints
union
select constraint_name, 50,99,'   tablespace indexes ' from user_constraints
union
select constraint_name, 60,99,'   storage(   ' from user_constraints
union
select constraint_name, 70,99,'   initial    ' from user_constraints
union
select constraint_name, 80,99,'   next       ' from user_constraints
union
select constraint_name, 90,99,'   pctincrease 0   );' from user_constraints
union
select constraint_name, 100,99, '  ' from user_constraints
order by 1,2,3;

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