使用变量在视图控制器之间切换

I've set-up multiple view controllers on of which is a game over scene. Now I'm able to switch between the scenes with buttons and then linking them afterwards by dragging them into each other. But how do I switch view controllers using variables. For example, I have an integer named lives and when it reaches zero, I want it to switch to the game over view controller.I've set-up multiple view controllers on of whi