With the release of Rocket.Chat 0.40.0 we are moving from requiring Node.js v0.10.44 to Node.js v4.8.4 LTS
If you used docker or one of our cloud installation methods: nothing to do, carry on!
First stop Rocket.Chat
The guides suggest the usage of n to manage Node.js version. Assuming that is what you used run:
sudo n 4.8.4