To calculate the percentage change between two months, you can use the following formula:

1. = (new value - old value) / old value

Let's say your old value is in cell A1 and your new value is in cell B1. You can enter the formula in cell C1 and then format the cell as a percentage. The result will be the percentage change between the two values.

To show the result as either a plus or minus, you can use conditional formatting. Here are the steps:

1. Select the cell with the percentage change (C1 in our example).
2. Go to the Home tab and click on Conditional Formatting.
3. Select "Highlight Cell Rules" and then "Less Than."
4. In the box next to "Less Than," enter 0 (zero) and select a red fill color.
5. Click "OK" to apply the formatting.
6. Repeat steps 3-5, but this time select "Greater Than" and enter 0 (zero) and select a green fill color.

Now, any negative percentage changes will be highlighted in red and any positive percentage changes will be highlighted in green.

I hope that helps!