change passwd

i want to change my sys and system passwd of a oracle database which is already in use.
can any one suggest me how to change that it is oracle 7.3

alter user system identified xxxx;
alter use sys identified yyyy;

To change password of any user use foolowing command

Alter user username
identified by password

kuckoo

alter user system identified by newsystempassword;
alter user sys identified by newsyspassword;