The last way may be easiest for you until you learn to double click, then the second method will be faster. But if there is no icon on the Desktop, you may need to use the Start , Programs sequence. You should now have at least two calculators on your Desktop. One is laid over the other. You have two windows open on your Desktop.
Windows are like pieces of paper on a desk.
You can have several windows on the Desktop at one time. Like pieces of paper, windows sometimes they get on top of each other and hide what's underneath. Move the pointer into the hidden, background calculator. Click and you'll see that it moves to the foreground. Click back and forth between the two calculators, changing the one that is active and in the foreground.