Use a title manager (like Tinfoil, DBI, or SX OS installer) to install the update over the base game.

NSP files are typically used for homebrew-enabled or modified consoles (running SX OS or Atmosphere) to install updates from unofficial sources. How to Use (Context: Modified Switch)

Extract the POKLGEV...zip file to get the .nsp update file.

If you can clarify the (Pokeland Legends or another one) and what problem you are trying to solve (e.g., crashing, missing DLC, failed update), I can give you specific installation steps or potential fixes.

Move the .nsp file to your Nintendo Switch SD card.

Used to update a pre-installed base game ( .nsp or .xci ) to a newer version to fix bugs or add content.