Move the extracted .nsp file to your Switch's SD card.
Use your preferred title manager (such as Tinfoil, DBI, or Goldleaf) to install the update. Undertale-NSP-UPDATE-ROMSLAB.rar
Ensure your custom firmware (CFW) is up to date to avoid installation errors. Always back up your save data before applying updates. Move the extracted
Use a tool like WinRAR or 7-Zip to extract the .nsp file from the .rar archive. Undertale-NSP-UPDATE-ROMSLAB.rar