Upgrade 128 GB SSD in DELL XPS 13 9350

Pretty much any M.2 SATA or M.2 NVMe SSD in the "2280" size (the most common) will work.  The only reason I say "pretty much" is that some Dell systems, including older XPS models, seem to have trouble specifically with the Samsung 970 Evo and Pro, even though they work fine with the 960 Evo and Pro.  The main difference between M.2 SATA and M.2 NVMe is performance.  M.2 SATA SSDs max out around 550 MB/s, whereas today's M.2 NVMe SSDs can exceed 3 GB/s.  I believe the XPS 13 9350 when shipped with a 128 GB SSD came with a SATA version, not NVMe, but all XPS 13 9350 systems support both SATA and NVMe regardless of what they came with from the factory.  However, be aware that in the XPS 13 9350, the NVMe interface is configured in firmware to run in power saving mode rather than max performance mode.  This is not a configurable option, and it means that NVMe SSD performance will be limited to roughly 1.8 GB/s -- but that's still over 3x faster than a SATA SSD.

If you want NVMe performance, I would suggest a Samsung 960 Evo.  There are several confirmed reports of that SSD working in your system.  If you're ok with just SATA speeds, look at the Samsung 860 Evo, although make sure you get the M.2 version in that case, since the 860 is also available in mSATA and 2.5" SATA versions, neither of which will fit your system.