Steam from a kettle or the warmth from a hairdryer will usually work to reshape a natural hair artist brush. Place it in the steam for a moment and then carefully reshape the tip with your fingers. Repeat a few times. Dip in gum arabic solution and allow to dry for additional re-shaping help.
Dipping in very hot water and reshaping with your fingers can sometimes help with a synthetic brush, but they can't always be reshaped.