To open the cover,u have to melt the glue thats holding it in place.Normally you pour boiling water on the edges to melt the glue,then pull the cover till it comes off.To install back, buy the hot glue gun and glue along the edges inside.Let it dry,then do the same alone the outside edge.U have to do this step very carefully or else your head lamp will fog up inside when it rains or when you wash your car!
Hope that helps..good luck