Update my phone

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.

iPhone 6 Plus, 12

iOS 12.5.7 is the latest iOS security update for iPhone 6 Plus

Go to Settings -> General -> Software Update -> Download and Install (if you haven't already done that).

iPhone 6 Plus can’t be update to a higher iOS (13 and later). 

If you have an outdated iPhone that cannot be updated to iOS 13 or later, many apps will not be able to download.

Outdated iPhone no longer meets the requirements for many apps.

Most apps require iOS 13.4 or later.                                

Considering a newer type of iPhone?

Read this before you buy a newer type iPhone.

is working too slow.

See the following Apple articles:

Connect to Wi-Fi on your iPhone, iPad, or iPod touch

Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support

If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network

If your iPhone or iPad won't connect to a Wi-Fi network - Apple Support

Check the route settings -> Recommended settings for Wi-Fi routers and access points

Recommended settings for Wi-Fi routers and access points - Apple Support

If you need help with your Wi‑Fi password

If you need help with your Wi‑Fi password – Apple Support (UK)

•  You need to sign in to your router.
• The Wi-Fi network name, or SSID (service set identifier), is the name your network that has a password.

If they are On, turn Off Hotspot & VPN.

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iPhone 6 Plus is > 7 years old, which also affects the battery life. The hardware remains old, so does the battery.

If it is still possible, have the battery replaced.

The battery can be replaced by Apple or an Apple Authorized Service Provider.

See this Apple article: iPhone Battery Repair & Replacement - Apple Support

If you are going to replace the battery, see -> Get your iPhone, iPad, or iPod touch ready for service

Get your iPhone, iPad, or iPod touch ready for service - Apple Support

The price is approx $ 69.

If you want to know exactly, call Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK) 

or 

call 1-800-MY-APPLE