How do u change ur directory using a shell script??

same way as you do it without a shell script.

i tried giving the command 'cd /home/username' in my shell script and after executing the script i'm back in the directory where i started..

cause you work in a subshell in your script. do something like this in your script:

#!/your/shell
cd /home/username
pwd
exit 0

I tried what u said.. But i'm still in the current directory and not in the one i specified in the shell script

You can change the directory. But it depends on what you are trying to achieve with your script.

Run your script as
When you run the script in this manner, all the changes you make to env variables will remain after your exit the script.

and what is the output of "pwd" in the script?