Since you are new at this, here are some details.
CHARLES and DPAP are different users in the database (user=schema, don't get hung up on these terms).
If they both have the CREATE SESSION privilege, then either of them could connect to the database. In your case, DPAP just happens to have the ability to "connect through" charles, so that using Charles's username and password, we can connect to DPAP.
There is no Charles[DPAP] user, and this syntax is never used in SQL. It is only used during login to indicate that one user is connecting through another.
The users are set up like this:
c:\sqlplus / as sysdba
SQL*Plus: Release 12.2.0.0.0 on Thu Sep 19 12:25:59 2019
Copyright (c) 1982, 2015, Oracle. All rights reserved.
SQL> create user charles identified by charles;
User created.
SQL> create user dpap identified by dpap;
User created.
SQL> alter user dpap grant connect through charles;
User altered.
SQL> grant create session to dpap;
Grant succeeded.
Then, to connect....
C:\>sqlplus charles[dpap]/charles
SQL*Plus: Release 12.2.0.0.0 on Thu Sep 19 12:33:08 2019
Copyright (c) 1982, 2015, Oracle. All rights reserved.
Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.0.1 - 64bit
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options
SQL> show user;
USER is "DPAP"
now, to change the CHARLES's password...back in the DBA session....
SQL> alter user charles identified by new_password;
User altered.
tack on "password expire" if you want.
and now....
C:\>sqlplus charles[dpap]/new_password
SQL*Plus: Release 12.2.0.0.0 on Thu Sep 19 12:43:01 2019
Copyright (c) 1982, 2015, Oracle. All rights reserved.
Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.0.1 - 64bit
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options