If we want to change the guest user password in E- Biz 11i or R12, the below steps can be followed.

STEPS

1. Login as the "applmgr"(or appserver user which is being used) Unix user and make sure the apps environment files are sourced.

2. Change the below parameter in Context file.
 Ensure the Guest password is entirely uppercase

AutoConfig variable name is "s_guest_pass"

3. Run AutoConfig.

4.  Verify the new guest password as follows:

  Login to sqlplus as apps user

 select fnd_web_sec.validate_login('GUEST','<NEW_PSWD>') from dual;

 this script should return 'Y'

5. Change the guest password used for diagnostics as described in Note 235307.1 "eBusiness Support Diagnostics FAQ and Troubleshooting Guide" and summarized below:

a. Login to applications as SYSADMIN user and choose "CRM HTML Administration" responsibility. (If you don't see this responsibility, it may be end dated for SYSADMIN user.)
b. Then follow this path and correct the GUEST password.
Settings : System : Properties
Self Service User
Settings
System
Properties > Advanced
Choose JTF from the "View" LOV.
Look for guest_password and correct it

6. Bounce(Stop / Start) Apache.


How to change Guest user password from Backend

set serveroutput on
declare
l_result varchar2(30000);
userid number;
BEGIN
l_result := fnd_web_sec.CHANGE_GUEST_PASSWORD('GUEST','<appspassword>');
dbms_output.put_line( 'Result = ' || l_result );
if l_result = 'N'
then
l_result := fnd_message.get();
dbms_output.put_line( 'Error stack = ' || l_result );
end if;
END;

How to troubleshoot issues with Guest user password

First check the password using the sql

select fnd_web_sec.validate_login('GUEST','ORACLE') from dual;

FND_WEB_SEC.VALIDATE_LOGIN('GUEST','ORACLE')
------------------------------------------------------------------------
N

If the output is N, we need to check for the error using below command

select fnd_message.get from dual;