It seems that the only way to upgrade the version of Node used by Meteor is to upgrade Meteor, according to the following:
You can run your production bundle with other node versions, it is just not guaranteed that it will work. In practice though, I've done this many times and have never run into any issues (always using newer versions of node).
In development meteor uses the node binary in
~/.meteor/packages/meteor-tool/METEOR_VERSION/mt-os.linux.x86_64/dev_bundle/bin/
where METEOR_VERSION is the version of meteor your project is using, e.g., 1.12.1. So you could go there and replace that binary with a symlink to another node executable if you want to and it will probably work for most newer versions of node.
In development: difficult. Your Meteor project uses its own embedded Node version, and trying to change it might also require reconfiguring the dev mode build and tooling...
In production: if you have control of the server environment, then yes, you can try another Node version, at your own risks regarding compatibility with Meteor code. However the latter checks that your Node major version is at least the one required by Meteor.
In Heroku: if you use the Meteor buildpack horse, then no. The buildpack configures your Heroku environment as required by the Meteor version of your project, including the Node version.