i have a HP Pavilion dv6-6b45eo notebook pc with AMD Radeon 6490m and i want a new graphic card. can i change it, and how?

The short answer:  no. 

Even if your laptop did happen to have a discrete card that could be replaced, you'd still have to find one that fit in the same power and thermal envelope as the card you're replacing not to mention fitting physically inside the case.

There have been attempts in the past at making some kind of portable graphics module that you could just plug in, but so far nothing has really worked out.

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