Server side validations for login page

Hi,

I have to do server side validation on log in page.
Once i logging with username and pwd, i am redirected to the next page
where i will the details of the logged user.
here before the page submits and goes to next page i would want to
change the values of the page and submit it

i mean hack it.I need this as i have a test case to validate this.

Please let me know how do i do this

Thanks
Geeta