can I replace a AMD Radeon R9 M290X 2GB GDDR5 with a AMD Radeon R9 M295 iMac 2015X 4GB GDDR5 in an iMac 2015

You can check at ifixit.com as I'm not familiar with the brand new iMacs, but generally, it was not possible unless the desired graphics card was offered as an option during the purchase process - in other words, the iMac was made to be able to work with both and the card was not soldered to the logic board (as a majority of the models are). Also, the new iMacs are even more difficult to disassemble because the screen is glued to the frame; you do realize that you may forfeit your warranty unless you have the work done at an authorized service provider.