I’m trying to download new 14.0 iOS on my phone but it shuts down before its completed. Any ideas?

Okay, it should be a 6s model but.... only 16GB ???

In any case it supports iOS 14.6, but since it is still running iOS 12, you might have difficulties to update it wirelessly (via WiFi). See also this support article:--> If your iPhone, iPad or iPod touch won’t update

You should try to update it using a computer running iTunes (PC) or iTunes/Finder (Mac).

If you don't have a computer, borrow one from your friends or ask for help at an Apple Store or Authorized Service Provider.

Regards

GIulio

The support article you came here from should provide you with the steps necessary to resolve this. If none of them worked, you will need to contact Apple Support directly for further assistance. You can do so using the "Get Support" link in the upper right-hand side of this forum's web page.

Are you sure your iPhone is a model 6s (not a model 6)?

Please go to Settings>General>About>Model Number and post it here.

Thanks

Giulio