Yes, the SSD in the Dell XPS 13 Plus is user upgradeable. The slot in your machine is an M.2 2230 or 2280 drive with a PCIe Gen 3x4 or PCIe Gen4x4 NVME interface.
I had an early warranty repair on mine, and noticed a standard SSD drive attached to the motherboard and it was accessible. The tech confirmed that the SSD is replaceable. RAM is definitely not, however.
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