It depends on which 15R model you now have. The current ones use U-type (ultra low power) processors that are soldered to the mainboard, not socketed -- these cannot be upgraded. If your current CPU model ends in "U" -- such as i5-3317U, this is the case.
Any upgrade of the CPU on the older models that are socketed voids the warranty and is not supported by Dell, so you're on your own.