This guide will walk you through both processes -- selecting a ringtone and using the silent mode in the new iPhone device.
Select a Ring Tone
Follow these steps to customize your iPhone's sound settings and select any ring tone you like to use as incoming call alert sound.
1. Tap on Settings to get started.
2. Scroll to and select Sounds under Settings.
3. Scroll to and tap Ringtone under Sounds and Vibration Patterns.
4. In the next screen, you will be prompted with the available ringtones pre-loaded in the new iPhone, including the default ringtone. Tap on the ringtone to listen to it.
5. Once you have found the ring tone you like, tap on it to select, and then tap Sounds.
You now have changed the ringtone on your new iPhone device. You can place a test call to see if that works. If you don't like it, then just follow the same steps to get through the ringtone options, then choose another ringtone you like.
Use Silent Mode
To use silent mode in the new iPhone 6 Plus, all you have to do is to slide the Ring/Silent mode key right or left. Doing so will turn silent mode on or off.
When the phone is in silent mode, all sounds in it are turned off. If you want to set it back to ring mode, slide the same switch again.
Ring/Silent Mode on iPhone 6 and iPhone 6 Plus not working/defective
Just a heads-up though, in case you will encounter any trouble using the ring/silent switch on your iPhone 6 Plus. The best thing to do is to have your iPhone replaced immediately because there's no available fix for it so far. Apple's way of fixing this issue is by replacing the unit. It's under warranty anyway, so there shouldn't be any problem having a replacement unit.