If you just placed the Invisible Shield on recently, you can peel it off and just rub the shield under water with some liquid hand soap as someone else stated. If the shield has been on for a couple of days, then it might have already cured which will cause the shield to stretch if you attempt to remove it.
To make more of the spray that they package with the Invisible Shield, just add a small drop of liquid hand soap to the tiny spray bottle and fill the rest with water, then just shake it.
If anything, Invisible Shield offers lifetime warranty, so you can always send your pieces in for a new shield, but this will take a lot longer.