I have had a chemical burn mark to which i applied lightening cream. the mark gradually faded at center but is not fading around edges. what to do?

Apply beyond border: It depends on how long you have used the cream -- you may need to continue for a while longer. Also, you may need to apply more of the cream to the border and even slightly beyond the border to ensure that it lightens. However, you will run the risk of the center of the lesion and the normal skin outside the lesion to become slightly lighter. Treating marks/scars can be very challenging.