I am trying to change user from within a script using the su command. But su command does not take the password as argument. So when i call su from my script, it displays the enter password prompt. But I want to hard code the password in the script itself and use it for the user change. Is it possible somehow? If not is there any other way to achive this (i.e. change the user)?

as a simple shell script you'd be putting your root password in plain text if you put it as part of the command (hard coding).... BAAAAAAAAAAAAD (even if setting strict permissions)

You could use sudo (man sudo) edit /etc/sudoers to allow your user to run that program that normally requires root, and without a password, so the command in your script is
and it runs

more than one user? setup a group in sudoers, add the users to the group.

What is the script trying to do?

As an after thought (i'm still new to Linux and sripting), maybe you could pipe the correct column value from /etc/passwd or the shadow file and it would work, maybe worth a try.

If you're trying to do something with root permissions, it probably should be in a script run as root (such as a startup script), rather than doing something funny with su or sudo.