Please specify the exam you are talking about.But for most of exam there is a forget password option in their official portal.So you can change your password by this option.

If you have forgot your password just find your account got to the reset password , a verification code or link would be sent to your backup phone no. Or mail 
Then click that link and reset your password

Hello,
You have to tell us that for which entrance exam you have forgot password.But for most of tgem,you need to visit login page and have to click on forgot password and after that enter your application form number and then you will receive an email to correct your password again.
Try to visit FAQ section of that website for more details.
Good luck.