Why won’t my iPhone update to the latest iOS?

Assuming your iPhone is a 6s model, still running iOS 13, that iOS version is very old to have it updated "wirelessly" (via wifi) installing 2 "major" versions like iOS 14 and iOS 15.

I suggest that you try to update it via computer (if you don't have a computer borrow one from your friends/neighbors, or ask for help at an Apple Store or Authorized Service Provider):

Learn how to update your device manually if you're using a Mac with macOS Catalina or later, or if you're using a Mac with macOS Mojave or earlier or a Windows PC.

If you still get no help, contact Apple Support directly, using this link:--> Get Support

Regards

Giulio

Roy_C_C wrote:

Trying to update to 15.1 IOS. It downloaded, but won’t install. Message comes up “unable to install, try again later”.

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