What is the cost for the word update?

in app store they have microsoft office 365 which is a subscription

if you wish the non subscription version then you can go here Software - Mac Accessories - Apple

they don't sell word as standalone though I believe only as part of the office pack

if you rather not pay anything then Home | LibreOffice - Free Office Suite - Based on OpenOffice - Compatible with Microsoft

is free and support reading and writing ms office file formats