Welcome to the Apple Support Communities
As it looks like Pages was installed from the App Store, you have two possibilities to uninstall the application:
1. Open Launchpad, hold Option key and press X button above Pages icon to uninstall the application.
2. Open Finder, select Applications on the sidebar and remove Pages.
After doing this, open App Store and purchase Pages, so it will be registered to your Apple ID and you will be able to update that app