Your Roku TV is designed to ensure it is always running the latest version of software. It will check to see whether new software is available for download during initial setup and whenever it is powered on, and then randomly every 24-36 hours. The download and installation are done automatically without ever interrupting use of your Roku TV. To receive automatic updates, make sure your Roku TV is connected to the internet.
If your Roku TV has been disconnected for an extended period of time, you can have it manually check for a software update.