npm install ignores an already installed module with fuzzy versioning whereas npm update updates it.

devDependencies are handled by npm install and update differently. That is npm install will install/update devDependenciesunless --production flag is added and npm update will ignore devDependencies unless --dev flag is added.

Npm install does more than handling your dependencies in package.json. You can manually install node-modules.

Set node-modules as global using npm install –g <name>

If you want you can install specific versions described by git tags.

Forcibly install using –force.

Npm install installs all packages listed in packages.json and their dependencies.

Npm install express installs only the express modules and their dependencies.