OPS$<SID>ADM password expire

Hi,

We are receiving warnings in system that our User OPS$<SID>ADM password is going to expire.

USERNAME EXPIRY_DATE PASSWORD

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OPS$<SIDADM>ADM 12-JAN-11 EXTERNAL

As I understand User is Externally identified so I was expecting that if I change password of User <SID>ADM at OS level,

On database level OPS$ User should be Updated but it did not get update and still its expiry date is same.

So please suggest how can we change its password.

Shivam