Step 1: Open the Windows Settings

To open windows settings, press the start button at the bottom left of your screen 

Next click the gear icon, highlighted below

Now you should have the Windows Settings opened

Step 2: Click on System

Step 3: Drag the monitors labeled 1 &2 into the correct position.

If you do not know which monitor corresponds to which number, press the identify button.

Step 4: Press Apply when the monitors are in the correct order