i have windows 7 how to install itunes

I believe that iOS 15 requires iTunes 12.12.0.6 or later, which in turn requires Windows 10. It won't work with Windows 7.

The last version of iTunes that works with Windows 7 is 12.10.11.2

See the iTunes installers section here:

Troubleshooting issues with iTunes for Wi… - Apple Community

Be aware that these old iTunes versions may not work with the newer iOS versions on your iPhone when you try to sync or backup.