How can I update my iPhone 6 Plus

You cannot pick and choose an iOS version. What you get is Settings > General > Software update > [All options are available here]

If iOS updates are no longer being released for your device, it typically means that the device has reached its end of life in terms of software support from Apple. In such cases, there are a few options you can consider:

1. Although major iOS updates may no longer be available, Apple may continue to release security updates for older devices. Ensure that your device is running the latest available version of iOS for its generation to maintain the highest level of security.
2. As iOS versions advance, some newer apps may not be compatible with older iOS versions. You may need to rely on older versions of apps or find alternative apps that are compatible with your device's iOS version.
   1. Ask the App developer to release the app for lower iOS versions also. Netflix & Amazon has done it.
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3. If your device is no longer receiving software updates, it may be a good time to consider upgrading to a newer device that is supported by the latest iOS updates. Upgrading to a newer device will ensure that you continue to receive the latest features, security updates, and app compatibility. Selection of a mobile model/brand is driv… - Apple Community

Remember to regularly back up your data to avoid losing any important information during the transition to a new device.

It's worth noting that the availability of updates and options may vary depending on the specific device model and its hardware capabilities.

You can check to see if any updates are available if you have a good Wi-Fi connecting and fully charged battery.

Tap Settings > General > Software Update

If an update is available, it will appear for you to install.

If you see a message that your iPhone is already up to date, there are no more updates available for the old iPhone 6 Plus. The Phone 6 Plus can only go as far as iOS 12.5.7.

Whether or not you are able to update any iPhone to a newer version of iOS will be dictated by the iPhone model - and the currently installed version of iOS. You can find this information here:

Settings > General > About

Not sure which iPhone you have? Identify your iPhone model - Apple Support

Here is a list of the highest version of iOS supported by recent models of iPhone (for simplicity - “generations” are shortened to a number), older devices being restricted in their ability to be updated:

iOS16.5.1

iPhone 14 (all models)

iPhone 13 (all models)

iPhone 12 (all models)

iPhone 11 (all models)

iPhone X, XR, XS

iPhone 8 (all models)

iPhone SE2/SE3

iOS 15.7.7

iPhone 7 (all models)

iPhone 6S (all models)

iPhone SE1

iOS 12.5.7

iPhone 6

iPhone 5S

iOS 10.3.4/3

iPhone 5/5C

If your iPhone is capable of update to a more recent version of iOS than is currently installed, these support pages outline both the available methods by which the update can be performed - and troubleshooting should you experience difficulties:

Update your iPhone, iPad or iPod touch – Apple Support

If you can't update or restore your iPhone, iPad or iPod touch – Apple Support

If your iPhone, iPad or iPod touch won’t update - Apple Support

iOS update and restore errors – Apple Support

Be aware that while an iPhone may be capable of update, in some circumstances you may need to use a computer to perform the update. To do so, you will need access to either a Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS). Ensure that your installed version of iTunes has been updated to the most recent version; the essential device drivers are included.

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

Update software on iPhone, iPad or iPod touch in iTunes on PC - Apple Support

If you have a beta profile installed, ensure this has been removed from your iPhone - as, otherwise, installation of production software updates may be inhibited. This support page provides the necessary information to remove the beta from your iPad:

How to uninstall iOS beta software - Apple Support

You should note that for your iPhone to install an available update, its battery must be charged to at least 50% capacity - and/or be connected to its Power Adapter.

Not sure which iPhone you have?

The OP told us, but your boilerplate response did not know this.

Bob Timmons wrote:

Not sure which iPhone you have?

The OP told us, but your boilerplate response did not know this.

Experience suggests that quoted information isn’t necessarily accurate.

Doesn't really matter what model. If the OP checks to see if any updates are available for his/her phone, they will appear if the phone is qualified for an update.

Thanks. Since you told us originally that you had an iPhone 6 Plus, we had no reason to assume otherwise.