The iPhone 6S is capable of updating to the latest iOS, iOS 15.6
The iPhone 6 without the S is capable of updating to iOS 12.5.5
If you have an iPhone 6S I would recommend that you try to update your phone via a pc or Mac
Update your iPhone using a Mac with macOS Catalina or higher
On a Mac with macOS Catalina or higher, you will use the finder in order to upgrade your iPhone. Make sure to Back up your device before starting the upgrade process.
On a Mac with macOS Catalina or later, you will follow the following steps:
- Open a finder window
- Connect your device to your Mac via a lighting to usb A or a lightning to usb c cable
- Select your device in the left sidebar of the finder window
- click general, then click check for update
- Click download and update.
- If asked, enter your iPhone pascode
Update your iPhone via a Mac running macOS Mojave or earlier, or on a Windows PC
On macOS Mojave or earlier, or on a windows machine, we won't use the Finder to upgrade your iOS
Instead, we will use iTunes. Make sure to Back up your device before doing any update or upgrade to your phone.
- Open iTunes
- connect your device to the computer with an lightning to usb a or a lightning to usb c cable
- Click device button in the iTunes toolbar
- In the summary tab, click check for update
- Click download and update, follow The on screen instructions
- If asked, enter your iPhone passcode.
See this link for further reference
Updating/upgrading your iPhone - Apple Community
