My 7 can’t download and update to iOS 15.8.1 so how can i update it

See how here --> Update iOS on iPhone - Apple Support

The phone should be plugged in to charge and connected to Wifi if trying over the air. If you use a VPN, turn it off or better yet, delete it completely as VPN on iPhone is useless.

Or follow the directions about how to use your computer to update your iPhone.

Follow these instructions:

Update your iPhone or iPadApple SupportOfficial Apple Support › en-us

If you are having trouble, you will need to work your way through this troubleshooting guide. Since there are about a dozen different reasons why the update is failing, and you won't know the real reason until the phone updates.

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

Plz my Software updated to 16.1

This thread is about an iPhone 7.

An iPhone 7 cannot download or install iOS 16.

You need an iPhone 8 if you want to run iOS 16.