Maybe Sony doesn't offer it? Then ask Sony why they don't.
Maybe your provider (say Verizon), if you bought a subsidised phone, doesn't offer it. Then ask them why.
You don't tell how you tried to update and why it didn't work. That ,makes it rather impossible to do any suggestion than: have a look at xda-developers to see if there's help there.
I am using sony xperia s(2013 model). I am trying to update
android new version (lollipop) os. But unable to update.
Please suggest any idea to update newer version
Thank you in advance!