A recent study managed to change subjects' opinions on faces from neutral to positive and negative alike through brain training. This new technique, once fully developed, can be used in helping with traumatic experiences and treating disorders such as depression.

Source: https://www.sciencenews.org/article/brain-training-can-alter-opinions-faces?tgt=nr
Photo source: By Macey et al., 2007 (Macey et al., 2007) [CC BY 2.5 (http://creativecommons.org/licenses/by/2.5)], via Wikimedia Commons