Changing canvas color?

I have created an image with a transparent background.

When i exported this image into a dark background, the jagged edge around the
images are visible.

How can i change the color of the canvas background so i can see the jagged edges of the images before i export?

I have had to open my image again in Photoshop but cannot find an option to change the canvas color?

Also when i have created an image with several layers. How is it possible to save the image so it is not flattened and i can open the image and amend each of the layers when required?

Sorry for sounding dumb

Can someone help please?