If you are going to change password of a role in oracle , there are two possible ways:
A- for none built in roles (sys,system) :
1- connect to oracle with sql plus or any other client tool
2- type :
3- old password will be asked first and after that you have to enter new password and its confirmation.
4- oracle will change the password ...
B- if you wante to change sys/system password , you have to work a little more :
1- connect to oracle
2- run this command :
alter user sys/system identified by newpassword;
3- restart your oracle service or your O.S
C- You can use Update command :
update sys.user$ set password='Newpass' where name='SCOTT';
it's needed to mention that sys/system passwords will change after system/service restart and before that still you can connect with old passwords .