Please follow this short step by step guide on how to change your existing theme in Windows 10.
First, right click on the desktop screen and select the personalize option. The control panel will automatically open to the personalization section of Windows 10. Here you will see a selection of themes in which to choose from.
Select the theme option you require and minimise the box you are in and it will then appear as an icon on the desktop (notice that the background picture will have already changed).
To change it again, you can now just right click the desktop theme icon and select personalize.
From this area, select the themes option along the left hand side and choose them settings and the same personalisation section we were in previously will appear.
To change it, do the same again, select the required theme, minimise the box, and the chosen theme will appear.
You can also right click on the desktop page and select next desktop background to keep changing to a different desktop picture (there will be several different pictures for each different theme).