How to invert unit step function?

[fahraynk]

I am trying to find out how to reverse the unit step function. The closest I could find is this sentence, which is more like a definition?

"if we want to reverse the unit step function, we can flip it around the y-axis as such: u(-t). With a little bit of manipulation, we can come to an important result:
$$u(-t)=1-u(t)$$

I completely get how 1-U(t) would be positive 1 until t=0 and then it becomes 0... But how does t change to -t?

 

[mfb]

The equation there is an identity. It can also be written as u(t)+u(-t)=1.

It does not change the function, but it tells you how you can change the function: u(-t) is the reversed function (by definition), but instead of this you can also use 1-u(t) because those two things are the same.