Posted by If you cannot wait for a fully autonomous offering, and you’ve jumped into the 20c preview release on Oracle Cloud, obviously the first thing you will probably be installing is Oracle Application Express.
Unlike autonomous, you’ll be installing it manually, which is a quick and easy process, and either in that installation or when adding ORDS later, you’ll be wanting to set the passwords for the public access accounts (typically APEX_PUBLIC_USER and APEX_REST_PUBLIC_USER).
Here’s what that looks like in Oracle Database 19c
SQL> show user
USER is "SYS"
SQL> alter user APEX_PUBLIC_USER identified by MY_SECRET_PASSWORD;
User altered.
And here is what it will look like in Oracle Database 20c 
SQL> show user
USER is "SYS"
SQL> alter user APEX_PUBLIC_USER identified by MY_SECRET_PASSWORD;
alter user APEX_PUBLIC_USER identified by MY_SECRET_PASSWORD
*
ERROR at line 1:
ORA-01031: insufficient privileges
Since Oracle 11, the control of user accounts has been improving, including controlling which accounts are explicitly related to the delivery of Oracle solutions (such as APEX, ORDS, Context, JVM and the like). In particular, the ORACLE_MAINTAINED column was added to DBA_USERS to let administrators know which accounts are managed by the database.
SQL> desc DBA_USERS
Name Null? Type
----------------------------------------- -------- ----------------------------
USERNAME NOT NULL VARCHAR2(128)
USER_ID NOT NULL NUMBER
PASSWORD VARCHAR2(4000)
ACCOUNT_STATUS NOT NULL VARCHAR2(32)
LOCK_DATE DATE
EXPIRY_DATE DATE
DEFAULT_TABLESPACE NOT NULL VARCHAR2(30)
TEMPORARY_TABLESPACE NOT NULL VARCHAR2(30)
LOCAL_TEMP_TABLESPACE VARCHAR2(30)
CREATED NOT NULL DATE
PROFILE NOT NULL VARCHAR2(128)
INITIAL_RSRC_CONSUMER_GROUP VARCHAR2(128)
EXTERNAL_NAME VARCHAR2(4000)
PASSWORD_VERSIONS VARCHAR2(17)
EDITIONS_ENABLED VARCHAR2(1)
AUTHENTICATION_TYPE VARCHAR2(8)
PROXY_ONLY_CONNECT VARCHAR2(1)
COMMON VARCHAR2(3)
LAST_LOGIN TIMESTAMP(9) WITH TIME ZONE
ORACLE_MAINTAINED VARCHAR2(1)
INHERITED VARCHAR2(3)
DEFAULT_COLLATION VARCHAR2(100)
IMPLICIT VARCHAR2(3)
ALL_SHARD VARCHAR2(3)
PASSWORD_CHANGE_DATE DATE
In 20c password management has been tightened for Oracle maintained accounts, of which the APEX accounts are included. Hence, to update their passwords, you’ll need to let the database know that you know you are tinkering with special accounts.
SQL> alter session set "_oracle_script" = true;
Session altered.
SQL> alter user APEX_PUBLIC_USER identified by MY_SECRET_PASSWORD;
User altered.
Then all that is left to do is to sit back and bask in the joy of Application Express on the latest version of the Oracle Database.
Learning is not a spectator sport 
This feature is is really creative….