Since I had to google it myself:
After finding the specific version you want, install it using npm install -g [package-name]@[your-version]
Example:
$ npm view npm dist-tags
{ latest: '1.4.20',
'1.4': '1.4.6',
'1.2': '1.2.8000',
'v1.5rc': '1.5.0-alpha-1' }
$ npm install -g npm@1.2