How can update my iPhone 6 to iOS 12.5.5

An iPhone 6 can only run as high as iOS 12.5.5

- Any iPhone that can't run iOS 15 can't run higher than iOS 12.5.5

- To see a list of 'iOS 15 compatible devices', click here --> https://support.apple.com/HT212712

- This list does not include the model you mention.

Here's a simple and fast way to see what is possible with your device, including updating it to the highest sub-version (e.g., 12.5.4 --> 12.5.5) of an iOS/iPadOS you may already have:

On your device, tap:

Settings > General > Software Update.

1. If it says your device is up to date there is nothing more you can do. If apps you are trying to use say you need a higher iOS version, you will need to buy a newer device if you want to run those apps or features.

2. If it says an update is available you will normally only be given one update selection which will be the highest available iOS version supported by your device. However, if you are running an iOS 14 version that is OS 14.5 or greater, you may see two options. Click on a link below for full instructions about how to update wirelessly, or using a computer (see links in document) if you have storage or connectivity issues. If the update listed is not equal to or higher than the iOS version you need to use a feature, you again will need to buy newer equipment if you want to use that feature.

Update your iPhone, iPad, or iPod touch. Click here --> https://support.apple.com/HT204204

Or this video:

"How to update your iPhone or iPad" click here --> https://youtu.be/csJAEyvvruY

Hello ~ The iPhone 6 cannot be updated passed iOS 12. You need a newer iPhone. Go to upper black line and click on iPhone to see newest choices. If you consider a second hand device please take a moment to read this:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~