What is happening if you're trying to install it? Do you have an iPhone 6S or newer? Otherwise your iPhone does not support iOS 15
If you're having an eligible iPhone and cannot update wirelessly please update via a pc or Mac instead
Update your iPhone using a Mac with macOS Catalina or higher
On a Mac with macOS Catalina or higher, you will use the finder in order to upgrade your iPhone. Make sure to Back up your device before starting the upgrade process.
On a Mac with macOS Catalina or later, you will follow the following steps:
- Open a finder window
- Connect your device to your Mac via a lighting to usb A or a lightning to usb c cable On the iPhone 15 series, use an usb c to usb c cable.
- Select your device in the left sidebar of the finder window
- click general, then click check for update
- Click download and update.
- If asked, enter your iPhone passcode
Update your iPhone via a Mac running macOS Mojave or earlier, or on a Windows PC
On macOS Mojave or earlier, or on a windows machine, we won't use the Finder to upgrade your iOS
Instead, we will use iTunes. Make sure to Back up your device before doing any update or upgrade to your phone.
- Open iTunes
- connect your device to the computer with an lightning to usb a or a lightning to usb c cable. On the iPhone 15 series, use an usb c to usb c cable.
- Click device button in the iTunes toolbar
- In the summary tab, click check for update
- Click download and update, follow The on screen instructions
- If asked, enter your iPhone passcode.
See this user tip I have written for more reference about updating
Updating/upgrading your iPhone - Apple Community
