In  Neural network when performing back propagation we get change weights when we get these values we have to either add or subtract the change of weights from the existing weights. So my question is how do we get to know when to add and subtract when I am doing the back propagation(while doing manually) it’s not like we plot each iteration to check whether it’s increasing or decreasing.
 Step by step back propagation example 48
In the above post he has either adding or subtracting the change in weights randomly there was no explanation for it.