Cognitive dissonance

From March workbook, between two choice: which is better to choose?
1. Giving up one of the beliefs
2. Increasing dissonance(conflicting thought in brain)
what is better to do?
1. To give up old beliefs, practicing new ones?
2. Feel discord by dissonance, receive it and wait until i forgot old beliefs?
and one more question, about how long time is needed to believe very unbelievable?