can i install samsung 970 evo on xps 15 9550

There have been some issues reported with the 970 Evo. On some Dell systems (but not all), the 970 Evo isn't detected even by the BIOS when it's installed, even though the 960 Evo works just fine. I can't remember if there have been reports either way about the XPS 15 9550, but I would search this forum for information before you buy anything. In terms of upgrading, if you're installing an SSD, then you'll be removing the 32GB SSD, which is fine. You could optionally replace the HDD as well with another HDD or a 2.5" SSD, or if you don't want to have a second drive, technically you could even replace your existing battery with the larger battery that comes with XPS 15 systems that don't have a 2.5" drive installed, since the larger battery option occupies the space that the 2.5" drive uses. Either way, after you get your NVMe SSD installed, you can either boot from Windows 10 installation media to perform a clean install and then use Dell Update to get the necessary drivers, or else try to use a Dell Recovery tool. I typically recommend the former option since Dell Recovery images might have outdated versions of Windows 10, so it can take longer to get current.