Samar,

I'm assuming here that you would like to transfer your Mathematica installation to a new machine:
in such a case please contact Wolfram Customer Support via

http://www.wolfram.com/support/contact/email

or call them at

1-800-WOLFRAM ext 2   (+1-217-398-5151 outside the US)

during Wolfram Customer Support business hours, Monday-Friday 8am-5pm, US Central Time:
they will reset your activation key, so you can reuse it when activating Mathematica on your new machine.

Corresponding Wolfram Support Article:
http://support.wolfram.com/kb/717

If on the other hand you would like to use Mathematica on additional machines, then please have a look into our store:

https://store.wolfram.com/catalog

where you can purchase further Mathematica licenses.

Hope this helps,
Peter