Login into the target server as ORACLE; assuming ORACLE is the O/S account that installed Oracle software and also that this account belongs to dba group.
export target ORACLE_SID
export target ORACLE_HOME
sqlplus /nolog
conn sys as sysdba
When prompted for password just hit Enter key.
At this point in time you are in and you can just alter user sys identifies by "YOURNEWPASS".
Good luck.