You can remove favourites in two ways. Either navigate to the title within the app, and press the star in the top right or go to the favourites tab, tap and hold a magazine for a few seconds until the options menu pops up, then select 'Remove from Favourites'.
Downloaded magazines are deleted automatically as you download the new titles. You can also remove the downloaded magazines manually. You can do this by clicking on the tab Downloaded. Press and hold a magazine for a few seconds so that the options menu appears, then select 'Remove Downloaded Data'.