You can delete session and scene files from Settings in the app. In Settings, you should see a 'Manage Storage' option. Under that, you should see delete 'All Sessions', meaning all the meditation files you have downloaded, or 'All Scenes'. Tap the option you would like to remove from your downloaded files.
Once deleted, sessions will be automatically re-downloaded the next time you listen to them. Scenes can be re-downloaded from our Sounds menu (tap the sound icon in the top right corner of the home screen to get there).