Can I swap my Dell xps 15 9550 hdd for a SSD only if so which SSD can for the slot and how can I transfer OS from hdd to new SSD
You can replace the hard drive or install an M.2 drive in addition to it. You can then clone or image the existing drive using Macrium Reflect (free edition will do fine).
Which SSD would fit the same slot of the existing Hdd. I don't wanna have a external drive I rather install in inside less hassle
Any 2.5" 7 mm or slimmer SATA SSD. You can also install one M.2 2280 SSD internally as well.