I believe the SSD chips are soldered onto the logic board and can not be replaced/upgraded. The only thing you can upgrade in a 2018 mini is the RAM.
You could purchase an external SSD and hook it up to either the USB-C or USB-A ports. I personally have a SanDisk Extreme Portable SSD, and it's fast enough for my needs (Logic sounds, some other 3rd party instruments, 4K video files, etc.).