I want to upgrade my iPhone to iOS 15

Go to Settings/General/About and note the iPhone Model Name, then check its version compatibility from this list:

• iPhone 8 or newer, including SE 2nd and 3rd editions, you can update to 16.4.1
• Any iPhone series 6S and 7, and original SE: You can update to 15.7.5
• iPhone 5S or 6: You cannot update to iOS 13 or higher; you can update only to iOS 12.5.7
• iPhone 5: You can update only to 10.3.4
• iPhone 5C: You can update only to 10.3.3
• iPhone 4S: You can update only to 9.3.6
• iPhone 4: You can update only to 7.1.2

For sure check your iPhone model & iOS version -> Go to: Settings -> General -> About.

If it is an iPhone that you can update to iOS 15.7.5, you can check by reading this Apple article:

iPhone models compatible with iOS 12 -> iOS 16.

iPhone models compatible with iOS 15.5 – Apple Support (UK)

You need about 6 GB device storage and good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

Go to Settings -> General -> Software Update -> Download and Install.

See the following Apple articles:

Update your iPhone, iPad, or iPod touch

Update your iPhone, iPad, or iPod touch - Apple Support (IN)

If your iPhone, iPad or iPod touch won’t update

If your iPhone, iPad, or iPod touch won't update - Apple Support

You can do the update via PC, see this Apple article: Update software on iPhone, iPad or iPod touch in iTunes on PC

Update software on iPhone, iPad or iPod touch in iTunes on PC – Apple Support (UK)

If you have a Mac see this Apple article: Update your iPhone, iPad, or iPod touch with the Finder

Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

Note: IF iOS 12.5.7 is the latest iOS security update for your iPhone, then you can’t update to a higher iOS (13 and later).