You cannot change only the processor, as it is soldered to the motherboard. You could remove the entire motherboard and replace it with a motherboard containing the processor that you desire, such as these from Parts-People. Follow the disassembly and reassembly steps in Dell's Service Manual.Look on YouTube for videos showing disassembly of your model. But, be cautious with them, as some use steps or methods that are not appropriate. Use the videos to add to your understanding of what to do, but follow the steps in Dell's manual. Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard.
Es posible cambiar el procesador de mi DELL Inspiron 15-3551? Gracias.  
Cambiar el procesador de DELL Inspiron 15-3551