问题 运行更新后，npm消失了

我正在关注一个教程 nettutsplus，并且在运行npm update -g npm之后，当我键入npm时，我的npm不再存在。这是错误。出了什么问题，我该如何解决？

localhost:~ macgeekalex$ npm update -g npm
npm http GET https://registry.npmjs.org/npm
npm http 200 https://registry.npmjs.org/npm
npm http GET https://registry.npmjs.org/npm/-/npm-1.3.1.tgz
npm http 200 https://registry.npmjs.org/npm/-/npm-1.3.1.tgz
npm http GET https://registry.npmjs.org/npm/1.3.1
npm http 200 https://registry.npmjs.org/npm/1.3.1
npm http GET https://registry.npmjs.org/npm/-/npm-1.3.1.tgz
npm http 200 https://registry.npmjs.org/npm/-/npm-1.3.1.tgz
npm ERR! error rolling back Error: EACCES, unlink '/usr/local/lib/node_modules/npm'
npm ERR! error rolling back  npm@1.3.1 { [Error: EACCES, unlink '/usr/local/lib/node_modules/npm']
npm ERR! error rolling back   errno: 3,
npm ERR! error rolling back   code: 'EACCES',
npm ERR! error rolling back   path: '/usr/local/lib/node_modules/npm' }
npm ERR! Error: EACCES, unlink '/usr/local/lib/node_modules/npm'
npm ERR!  { [Error: EACCES, unlink '/usr/local/lib/node_modules/npm']
npm ERR!   errno: 3,
npm ERR!   code: 'EACCES',
npm ERR!   path: '/usr/local/lib/node_modules/npm' }
npm ERR! 
npm ERR! Please try running this command again as root/Administrator.

npm ERR! System Darwin 12.3.0
npm ERR! command "node" "/usr/local/bin/npm" "update" "-g" "npm"
npm ERR! cwd /Volumes/Prodigy RAIDER PRO/Users/macgeekalex
npm ERR! node -v v0.10.12
npm ERR! npm -v 1.2.32
npm ERR! path /usr/local/lib/node_modules/npm
npm ERR! code EACCES
npm ERR! errno 3
npm ERR! stack Error: EACCES, unlink '/usr/local/lib/node_modules/npm'
npm ERR! 
npm ERR! Additional logging details can be found in:
npm ERR!     /Volumes/Prodigy RAIDER PRO/Users/macgeekalex/npm-debug.log
npm ERR! not ok code 0