Most of the logic for the bot finding who to shoot is done within C++. This Service(BotSearchEnemyLOS) calls functions within the ShooterAIController, which is a C++ class, so you would either need to change the logic within ShooterAIController or make your own logic within Blueprints.
I would recommend using AI Perception and EQS for creating your custom bot logic. If you don’t know how to use these, Epic has a great tutorialon how to use them.
Alternatively you could delete BotSearchEnemyLOS from the BotBehavior tree. Then create a new AI controller that extends from ShooterAIController. Then set the bot’s enemy to the player (like I did in the BotSearchEnemyLOS Service) with the Begin Play event. Once again this isn’t the best way to do this, but it is a quick way to get what you want. Like I said before, I recommend using the AI Perception and EQS to get the Bot to do what you want.