What is the latest iOS version that my iPhone 6 can run?

dk ~ What version of iOS is your iPhone 6 running?...

Settings > General > About > iOS Version

Be aware that iPhone 6 cannot be updated beyond iOS 12:

iOS 13 and iPadOS are compatible with these devices

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The tags appended to your post suggest that you may have an iPhone6. The iPhone6 cannot be updated beyond 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 iPad 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:

iOS16.6

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 15.7.8

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.