Please I want to update my phone to iOS 15.8.1

See how here:

Update your iPhone or iPad - Apple Support

If you can’t perform the update wirelessly, see this:

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

Razgurung wrote:

iPhone 11 I want to update iOS 15.3.1 how

Apple does not typically permit intermediate iOS updates, unless there’s just been a big iOS update and you can sometimes then get offered both a point maintenance update for the now-previous iOS version or an update to the next version.

When updating, you typically get the latest available iOS for the particular iPhone.

You’ll (currently) get iOS 17.3.1 when you update an iPhone 11.

If an app or service minimally needs iOS 15.3.1 and doesn’t work on iOS 17.3.1, contact the app vendor.

If your iPhone is somehow still offering iOS 15.3.1 from ~two years ago (and is failing to install it), delete the downloaded iOS update, and re-update to get iOS 17.3.1. To delete an older and “stuck” update, Settings > General > iPhone Storage > find the downloaded iOS 15.3.1 update, and delete it. Update again.

iPhone 11 I want to update iOS 15.3.1 how

Razgurung wrote:

iPhone 11 I want to update iOS 15.3.1 how

For an iPhone 11, you can update it to iOS 17.3.1. You cannot update it to 15.

See the links posted above from Chattanoogan for guidance.