You can install multiple versions of npm by installing multiple versions of node. To do this, you can use nvm-windows. Each different installation of node installs a different version of npm
To use a different version of node and nvm in a project, you simple need to switch:
nvm use 5.0
You can also create a .nvmrc file containing the version number in the project root directory. nvm will use the specified version of node for that project.