This is a good opportunity to execute a system restore. You can restore your iPhone 6s using iTunes, but backup your device first. To proceed with iTunes restore, follow these steps:

1. Using the USB connector that came with your iPhone, connect it to your computer.
2. On your PC, launch iTunes. Make sure you have the most recent version of the iTunes app.
3. Wait for iTunes to detect your iPhone.
4. On your device, tap the option to Trust This Computer if offered.
5. In iTunes, choose your iPhone 6s plus from the list of available devices.
6. In iTunes, choose the option to Restore Backup.
7. Choose the most appropriate iOS backup to utilize. You may look up the backup file's date and size.
8. Wait for the restoration time to expire after clicking the Restore button.
9. After the computer has rebooted, keep your device connected to it and wait for it to sync with it.
10. You can now unplug your iPhone after the sync is complete.