1. connect to db:

sqlplus /nolog
connect / as sysdba;

2. To alter the password expiry policy for a certain user profile in Oracle first check wich profile the user is in using:

select profile from DBA_USERS where username = '<username>';

3. Then you can change the limit to never expire using:

alter profile <profile_name> limit password_life_time UNLIMITED;

If you want to previously check the limit you may use:

select resource_name,limit from dba_profiles where profile='<profile_name>';

4. then unlock the user

ALTER USER <username> ACCOUNT UNLOCK;
OR
ALTER USER <username> identified by <password> account unlock;