Go to Settings/General/Software Update while connected to power and a WiFi network. The update takes about 5 minutes.
For safety be sure you have a current iCloud backup. 99% chance you won't need it, but it's best to be safe.
There is also a very small chance that something will go wrong. It never has for me, but I do see posts from people who have had the update fail. I think the chances are very small if you have power and a good Internet connection, but if the update fails you will need a computer to recover the phone.