How to remove Apps in iTunes?

Hey,

I changed my macbook quite often lately and somehow messed up my iphone/ipad apps. So when i try to sync them, it says they couldn't be found. I am sure i deleted the old itunes directory by accident. Now when i try to download the app again it tells me i already have it.
How can i remove those apps from my purchases, so i can re-download or re-purchase them again?

Thanks