If you own the Japanese physical cart but want to preserve its update as an NSP (for offline installation on a second Switch), you can using the homebrew app nxdumptool .
Transfer the NSP file to your microSD card or prepare it for USB installation. final fantasy pixel remaster switch nsp update install
How to Install Final Fantasy Pixel Remaster Updates on Switch If you own the Japanese physical cart but
If you don't want to keep swapping your SD card, you can install directly from your PC. final fantasy pixel remaster switch nsp update install