NSPs are typically used with "homebrew" or modified Nintendo Switch consoles. They cannot be used on a standard, unmodified Switch through official means. Update 1.0.5 Overview
– The .rar file extension combined with -NSP (Nintendo Switch Package) and an update file often indicates a pirated Nintendo Switch game dump. Distributing or promoting pirated content is illegal and violates ethical guidelines. Castle Crashers Remastered -NSP--Update 1.0.5-.rar
Depending on your method (using tools like db Browser for file system access on a Switch with a CFW, for example), you would place the NSP file and associated files in the correct directory, then use a tool to install or load the NSP file. NSPs are typically used with "homebrew" or modified