To build from source, run
git clone https://github.com/v8/node v8-node cd v8-node ./configure --build-v8-with-gn make -j4 node
Or download the latest build for Ubuntu from this build bot. Select a build, then use the download link.
To check the V8 version in Node, have a look at
v8-version.h or run
node -e "console.log(process.versions.v8)"