Say, the current version is 1.3.2 and you want to release 1.4.0 with some prereleases in between
$ npm version
1.3.2
And you want to make a prerelease before releasing a minor change
$ npm version preminor
1.4.0-0
Then you've made some changes and want to make another prerelease
$ npm version prerelease
1.4.0-1
And now finally release a new minor change
$ npm version patch
1.4.0