What are some ways to we can reduce aggression?

I am going to assume that the question is asking for healthy ways to reduce aggression. I am also going to assume that medication is not an option (although it can be effective).
Probably the best thing that a person can do to help curb his/her aggressive tendencies is to understand what triggers the anger and aggression in the first place. Once that trigger can be identified, the person can actively seek to avoid behaviors and interactions that would normally trigger the aggression. If game night gets too competitive, consider changing the type of games being played or completely changing the type of get together.
Another possible solution is to take a "time out." The result is that you are taking a break from the frustrating situation. You are allowing yourself time to calm down without any additional stressors being added to the situation.
Finally, a person can find constructive ways to release the pent up aggression. A great method of doing this is physical exercise. Go to the batting cages and hit the stitching off the pitches, or kick a soccer ball around. Going for a run is a good idea too. The benefits of exercise come in two forms. First, the exercise rids you of that energetic, aggressive energy. Second, the exercise releases "feel good" hormones like endorphins and dopamine. Both of these create a sense of euphoria.
http://web.csulb.edu/~tstevens/b-anger.htm

https://www.healthcentral.com/article/8-ways-to-decrease-your-childs-aggressive-behavior

https://www.webmd.com/depression/guide/exercise-depression