How to change forgotten password?

If you have forgotten your password for the SYS or SYSTEM user and wish to change it, log onto the server locally and navigate to the command prompt. Type the following:

set ORACLE_SID=<SID>
sqlplus /nolog
connect / as sysdba

If you receive an error regarding insufficient privileges, or it prompts you for a password, modify the sqlnet.ora file (usually located in $ORACLE_HOMEnetworkadmin) by setting SQLNET.AUTHENTICATION_SERVICES=(NTS) and try again. Once you've logged into SQL*Plus successfully as SYS on the local server, change the password as follows:

alter user SYS identified by xxxxx;
and/or
alter user SYSTEM identified by xxxxx;