First, I checked my npm version as following
$npm -v
2.10.1
then I tried updating it via sudo npm install npm@latest -g and checked it again, it still displayed as 2.10.1, the latest version should be 4.5.0 though.
UPDATE I know some of you might think this is a duplicated question, however, I tried the some of ways described here, and then encountered the issue. still can't figure out how to solve it.