If you want to change a user's password, you can log in as SYSTEM and use the ALTER USER command as shown in the following example:

>.insqlplus /nolog

SQL> connect SYSTEM/globalguideline

Connected.

SQL> ALTER USER DEV IDENTIFIED BY beginner;

User altered.

Note that ALTER is SQL statement, so you need to terminate it with ";". This command resets DEV's password to "beginner".