SQL> select INSTANCE_NAME from v$instance;
INSTANCE_NAME
----------------
+ASM

SQL> ALTER USER sys IDENTIFIED BY <new_password> REPLACE <old_password>;
ALTER USER sys IDENTIFIED BY <new_password> REPLACE <old_password>
*
ERROR at line 1:
ORA-01109: database not open

The following error also might occur:
SQL> alter user sys identified by ;
alter user sys identified by
*
ERROR at line 1:
ORA-01031: insufficient privileges

$ export ORACLE_SID=+ASM
$ asmcmd
ASMCMD> passwd sys
Enter old password (optional): ******
Enter new password: ******

ASMCMD> orapwusr --modify --password sys

Enter password: ******
ASMCMD> exit