npm update -g makes my npm gone

I man on Mac OSX and want to perform a npm update to update all globally installed stuff.

After executing npm update -g, I got a lot of erros and npm is gone, like this:

localhost:~ myname$ npm
-bash: npm: command not found

I Googled but don't remember how I installed node. It seems it's not installed via brew, so that I can't use brew to un/reinstall node/npm.

-- UPDATE --

After checking the comments on some other links I decide to just reinstall node and my npm is back. But when I try to use npm install -g npm to update again, the issue returns. Here is the full stuff from my terminal:

localhost:~ myname$ npm install -g npm
npm ERR! Darwin 14.1.0
npm ERR! argv "node" "/usr/local/bin/npm" "install" "-g" "npm"
npm ERR! node v0.12.1
npm ERR! npm  v2.5.1
npm ERR! path /usr/local/lib/node_modules/npm
npm ERR! code EACCES
npm ERR! errno -13

npm ERR! Error: EACCES, unlink '/usr/local/lib/node_modules/npm'
npm ERR!     at Error (native)
npm ERR!  { [Error: EACCES, unlink '/usr/local/lib/node_modules/npm']
npm ERR!   errno: -13,
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! error rolling back Error: EACCES, unlink '/usr/local/lib/node_modules/npm'
npm ERR! error rolling back     at Error (native)
npm ERR! error rolling back  { [Error: EACCES, unlink '/usr/local/lib/node_modules/npm']
npm ERR! error rolling back   errno: -13,
npm ERR! error rolling back   code: 'EACCES',
npm ERR! error rolling back   path: '/usr/local/lib/node_modules/npm' }

npm ERR! Please include the following file with any support request:
npm ERR!     /Users/myname/npm-debug.log
localhost:~ myname$ npm
-bash: /usr/local/bin/npm: No such file or directory

What should I do?

Thanks!