How to alter price in checkout page and when submit order I have a discount price for product. I want alter price of product  using discount price if user has voucher.
How to alter price of page cart, cart/checkout and when user submit order.
Please help me!
You could use a HOOK for your needs, just read a little bit about hooks, and then create a hook for your content type and change that price.

You can check here how to first create a module and then a hook

Hooks in drupal tutorial