Well , this is a simple command , so there is not much to be said about it , basically you need to go into SQLPLUS , to do this:
cd into $ORACLE_HOME , then cd into directory bin , on directory bin run:
./sqlplus /nolog
then type: connect / as sysdba
then run the alter command referring to the user you want to change the password to:
alter user sys identified by new_password;
In this case you can replace sys with the user you want to change the password to , and new_password to whatever secret you want to use.
Hope it helps
Felipe