Have you set up your ledger according to the instructions (copied seed, set pin .. etc)? Be sure that you have copied your seed somewhere safe before updating anything.
You can check your firmware version by going to Settings -> Device -> Firmware. If you are running version 1.3.1, you don't need to do anything. So if your ledger is new, chances are it is already updated.
If you don't have the latest firmware, You will have to manually update your firmware by downloading the chrome ledger manager desktop application, opening the application once installed, connecting your ledger to your computer, entering your pin, and using the ledger manager desktop application to manually update the firmware. This process will wipe your device so be sure you have a copy of your seed written down before this.
Here is an official guide:
https://ledger.zendesk.com/hc/en-us/articles/115005165409-How-can-I-update-my-Nano-S-