Incorrect iPhone settings, particularly those related to iOS network features, might also cause issues while establishing an iCloud backup. If you're having trouble backing up your iPhone 6s to iCloud after making changes to your iPhone settings or downloading new applications or software updates, you may try resetting all of your device's settings. Despite the fact that you will lose your wireless network information, including your Wi-Fi network passwords, you will be able to restore your network settings to their original state and remedy any errors created by the wrong configuration. Here's how you do it:

1. From the Home screen, choose Settings.
2. Select General.
3. Tap Reset after scrolling to the bottom of the menu.
4. Choose Reset All Settings from the drop-down menu.
5. If asked, confirm the action.
6. Wait for your iPhone to finish the settings reset, and then your iPhone will be ready to use