The apps list in iTunes when you have connected your iPad and are selecting what you wish to sync to the device.
If you've chosen to download the app from within the iPad and synced back choosing to "Transfer" data back to your computer you can just 'delete' the app by tap and holding until you get the wiggling icons. Then tap the "X" to delete the app you wish.
In the event you don't find the app in your iTunes list of applications, Apple lets you redownload it without them charging you (at least as many times as I've tried but there may be a limit to how many redownloads you get per app). Just browse/search for the app you already purchased. Click/tap buy and once you've authenticated to your account it should pop up a notice of already purchased item and download for free.