You have to remove the old heatsink during the graphics card replacement.  Then installing new heatsink along with new graphics card.  However, the whole process is tedious.  It requires planning and organize small parts with proper sequencing of removal and installation of components and chassis.  Review the linked videos ahead of time and using the service manual for reference.
Hi, I have a dell precision m6800 with a 4610m processor.  Will I be able to directly swop my amd firepro m6100 to the gtx 980m 
M6800 Gpu upgrade