All purchases are considered final, but you can try contacting iTunes support and see if they will refund or credit you : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption
There is no Word app from MS in the store, only tutorials. If you want to be able to edit/create word documents on your iPad then the options include Apple's Pages app and from third-parties apps such as Documents To Go ('premium' version) and QuickOffice Pro HD (but check their iOS version on the left-hand side of their requirements pages in the store, as they might need a higher version than you have if your 4.3 tagline is correct).