why cant i update my iphone

Hello,

When updating, your device will update to the highest version currently available that it will support. Here are two different ways to ensure the highest iOS that your device will support is installed on your device.

For instructions about updating your iPhone, iPad, or iPod touch wirelessly, click here --> Update your iPhone or iPad - Apple Support

Here are instructions about how to update your iPhone, iPad, or iPod touch with the Finder on a Mac computer --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Your iPhone 5 can only update as far as iOS 10.3.4

If your Apple mobile device won't update, read this support article --> If your iPhone or iPad won't update - Apple Support