How do I update iOS 9 to iOS 12

I have a iPhone 6s+ on ios 9.3.5. I'm trying to update it to iOS 12.x and my phone will not update. It says in order to update I have to plug up my phone up to iTunes because a wireless update is not available. Fine. I plug up my phone, go to iTunes (which is up to date), click the update button and nothing happens. I double click it and a little window will pop up for a second saying "Contacting the iPhone software update server" with a little load bar and in a blink of an eye it's gone. How do I update my phone?