iMac 27 mid 2015 SSD Upgrade

I have an 27 inch 5K iMac from mid 2015 (iMac 15,1) with 1Tb HDD (without Fusion Drive). I would like upgrade it to an SSD. After some research, I can't find any information about PCI Express slot from the motherboard. I would like to know if my iMac has a PCIe slot ? Also I would like to know what type of SSD is compatible with my iMac and what should I buy, what are the number of pins required for this model. It will be great if I can use PCI Express slot and leave my HDD. Thank in advance.