One note for people who might not have the set password for sysdba or sys and regularly use a third party client.
Here's some info about logging into command line sqlplus without a password that helped me.
I am using fedora 21 by the way.
locate sqlplus
In my case, sqlplus is located here:
/u01/app/oracle/product/11.2.0/xe/config/scripts/sqlplus.sh
Now run
cd /u01/app/oracle/product/11.2.0/xe/config/scripts
./sqlplus.sh / as sysdba
Now you need to connect to database with your old credentials.
You can find Oracle provided template in your output:
Use "connect username/password@XE" to connect to the database.
In my case I have user "oracle" with password "oracle" so my input looks like
connect oracle/oracle@XE
Done. Now type your new password twice.
Then if you don't want your password to expire anymore you could run
ALTER PROFILE DEFAULT LIMIT PASSWORD_LIFE_TIME UNLIMITED;