If you need to test your package or occasionally need a beta or next version.
First you can bump the version to the next major version for example with:
npm version 1.0.0-beta.0
If you are using git this will commit the changes and tag the commit in this case with 1.0.0-beta.0.
After that you can publish your package with the flag --tag beta, which will upload your package with the tag @beta instead of @latest.
npm publish --tag beta
The tag can be pretty much everything and will not affect the stable versions. This means, you need to install this tagged version explicity with npm install my-package@1.0.0-beta.0 or npm install my-package@beta. The latter will install the latest version in your beta “branch”.