Problems in upgrading IOS on my Iphone 5s

Hi,

I am using Iphone 5s with IOS version 7.1.2(11D257).

Now I want to upgrade my IOS version to IOS 9.2.1 (the latest available)

In Settings section I can see the updates available of size 1.2KB and as I try to update the same it download data to certain range and then stops downloading and nothing happens ahead.

As per my knowledge if I am upgrading the data from IOS7 to IOS9 then data should more.

Can anyone help me in updating my IOS.

Regards,

Bhushan