Since you've asked in the iWork for iOS forum I'm going assume you're referring to Pages, Numbers or Keynote for an iPad or iPhone or iPod Touch. What account do you use to buy from iTunes? Open iTunes on your computer & click Apps in the left hand column. Then, in the lower right of the window, click on check for updates. You will then be presented with any apps that have updates & get a window asking you to log in.
If you're not asking about iOS apps, please post back & explain further especially what apps you need to update & how you bought them in the first place.