Can I upgrade the graphic card on my iMac(late 2013)?

Hello!

I was thinking of upgrading the graphic card on my iMac from 2gb to 4gb. Tell me if that is possible and how can I do that? Do i have to send the iMac at applr service centre? Below are the full specifications of my iMac.

iMac 27inch(late 2013)

3.4 GHz i5 quad core processor,

8gb Ram,

Nvidia Geforce GTX 775M 2GB(currently installed).

Any help would be appreciated.

Regards,

Shivank