How to update to iOS 15.7.5?

The tags appended to your post suggest that you have an iPhone6. The iPhone6 is capable of update to iOS 12.5.7; it does not support iOS13 or later. If you have an iPhone6 as indicated - and you need to update iOS - you have no alternative other than to replace your iPhone with a much newer (or current) model.

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:

iOS 17.0.3

iPhone 15 (all models)

iPhone 14 (all models)

iPhone 13 (all models)

iPhone 12 (all models)

iPhone 11 (all models)

iPhone XR, XS (all models)

iPhone SE2/SE3

iOS 16.7.1

iPhone X

iPhone 8 (all models)

iOS 15.7.9

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