Percentage Change
Subtract the old from the new, then divide by the old value. Show that as a Percentage.
Comparing Old to New
Change: subtract old value from new value. Example: You had 5 books, but now have 7. The change is: 75 = 2. 

Percentage Change: show that as a percent of the old value ... so divide by the old value and make it a percentage: So the percentage change from 5 to 7 is: 2/5 = 0.4 = 40% 
Percentage Change is all about comparing old to new values. See percentage change, difference and error for other options.
How to Calculate
Here are two ways to calculate a percentage change, use whichever method you prefer:
Method 1
Step 1: Calculate the change (subtract old value from the new value) 
Step 2: Divide that change by the old value (you will get a decimal number) 
Step 3: Convert that to a percentage (by multiplying by 100 and adding a "%" sign) 
Note: if the new value is greater then the old value, it is a percentage increase, otherwise it is a decrease. 
Method 2
Step 1: Divide the New Value by the Old Value (you will get a decimal number) 
Step 2: Convert that to a percentage (by multiplying by 100 and adding a "%" sign) 
Step 3: Subtract 100% from that 
Note: if the result is positive it is a percentage increase, if negative, just remove the minus sign and call it a decrease. 
Examples
Example: A pair of socks went from $5 to $6, what is the percentage change?
Answer (Method 1):
 Step 1: $5 to $6 is a $1 increase
 Step 2: Divide by the old value: $1/$5 = 0.2
 Step 3: Convert 0.2 to percentage: 0.2×100 = 20% rise.
Answer (Method 2):
 Step 1: Divide new value by old value: $6/$5 = 1.2
 Step 2: Convert to percentage: 1.2×100 = 120% (i.e. $6 is 120% of $5)
 Step 3: Subtract 100%: 120%  100% = 20%, and that means a 20% rise.
Another Example: There were 160 smarties in the box yesterday, but now there are 116, what is the percentage change?
Answer (Method 1): 160 to 116 is a decrease of 44. Compared to yesterday's value: 44/160 = 0.275 = 27.5% decrease.  
Answer (Method 2): Compare today's value with yesterday's value: 116/160 = 0.725 = 72.5%, so the new value is 72.5% of the old value. Subtract 100% and you get 27.5%, or a 27.5% decrease. 
Why Compare to Old Value?
Because you are saying how much a value has changed.
Example: Milk was $2, now it is $3 ... did it rise $1 compared to $2 or $3 ?
We compare to the original $2 value, so we say the change is $1/$2 = 0.5 which is a 50% increase.
The Formula
You can also put the values into this formula:
New Value  Old Value 
× 100% 
Old Value 
(The "" symbols mean absolute value, so negatives become positive)
Example: There were 200 customers yesterday, and 240 today:
240  200 
× 100% = (40/200) × 100% = 20% 
200 
A 20% increase.
Example: But if there were 240 customers yesterday, and 200 today we would get:
200  240 
× 100% = (40/240) × 100% = 16.6...% 
240 
A 16.6...% decrease.
How to Reverse a Rise or Fall
Some people think that a percentage increase can be "reversed" by the same percentage decrease. But no!
Example: 10% of 100
A 10% increase from 100 is an increase of 10, which equals 110 ...
... but a 10% reduction from 110 is a reduction of 11 (10% of 110 is 11)
So we ended up at 99 (not the 100 we started with)
What happened?
Because the percentage rise or fall is in relation to the old value:

How to do it properly
To "reverse" a percentage rise or fall, use the right formula here:
To Reverse:  Use this Percent:  Example 10% 

An "x" percent rise:  x/(1+x/100) 
10/(1+10/100) = 10/(1.1) = 9.0909... 
An "x" percent fall:  x/(1x/100) 
10/(110/100) = 10/(0.9) = 11.111... 
Or use this handydandy calculator (just type in a value and click in the other box) 