what does keep change to change means?

I think what you are talking about is a double negative. For example, If you have 5 - -3. It is the same as 5 + 3. So 5 minus negative 3 is really just 5 plus 3.

i have A+ in every subject and i dont want to get lower then A in math, tommorow is test,

Hard to believe someone who can't spell: TOMORROW !

Your question requires some context...what subject are you studying ? How about a sample problem ?

It is a little hard to tell what is going on without an example. Perhaps it is something like this.
.. x + 6 = 3x - 4
There are both x-terms and constants on both sides of the equation. We want an equation with constants on one side and x-terms on the other side.

We see that x is added on the left (and on the right). We are concerned with the added x-term on the left because it is the smallest of the x-terms in the equation. We can "undo" that addition by adding the opposite of that x-term, that is, by adding -x (also known as subtracting x). We must do that to both sides of the equation.
.. x - x + 6 = 3x - x - 4 ... -x added to both sides
.. 6 = 3x - x - 4 ... Collect terms on the left. This looks like we have changed the +x on the left to -x on the right.
.. 6 = 2x - 4 ... finish collecting terms.

Now, we have a -4 on the right that we don't want. We can undo the operation of subtracting 4 by adding 4, that is by adding the opposite of -4. We must do that to both sides of the equation.
.. 6 + 4 = 2x - 4 + 4 ... +4 added to both sides
.. 6 + 4 = 2x ... Collect terms on the right. This looks like we have changed the -4 on the right to a +4 on the left.
.. 10 = 2x ... finish collecting terms. Constant on one side, x-term on the other.
.. 10(1/2) = 2x(1/2) ... multiply (both sides) by the multiplicative inverse of the coefficient of x.
.. 5 = x
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Some people will tell you to solve an equation by changing an added term on one side to a subtracted term on the other side, and vice versa. I prefer that you learn it as adding the same thing to both sides, where the thing you are adding is the additive inverse (opposite) of the thing you don't want. This way, you drill into your head that the same operation must be performed on both sides of the equation. Of course, the additive inverse is that which makes the sum zero, so the unwanted term becomes zero (disappears) when you add its inverse.

There are different reasons to change signs.

Distributing a negative factor.
-1(x-3) = -x --3 = -x+3 = 3-x

Reversing the sign of an equation
-3x+y=-2
3x-y=2

Evaluating the quadratic formula?

We need more details to answer this properly.