I’m not able to update test user’s password in the test script

anything is changeable in RPT, if it's black and white (not flagged as a dataset candidate or substituted from a reference) just type in the new one

Best practice for passwords is to have them in a dataset, then substitute from the dataset

Then once working the dataset can be encrypted.

Here's a blog posting I wrote that will sub a password from a dataset and can be applied to multiple tests

http://www.testworkbench-community.com/blogs/changing-a-hard-coded-password-in-multiple-tests-in-performance-tester