Pixelmator
Changing a Subject's Background

Using a Mask to Change the Background

The easy down and dirty way to replace the background in any photo editor is to load both the image and the background you want to use and use the Remove Background tool to take away the image's original background so that the new background shows underneath it and that works pretty well most of the time

But by far the best way is to load the two images with the new background underneath your main image and to click Edit in the menu bar and the choose Select and Mask

You'll then be looking at a totally yellow screen so click the Select Subject button and the subject will be visible with the background remaining yellow but if there are parts of your subject that are also yellow you'll want to click the Minus symbol under the the Basic Brush button and paint over the yellow parts of your subject then click the Apply button at the bottom of the screen

Now that you have have just your subject and a clean erased background that you might think of as a clear glass pane it's time (if you haven't already) to load your desired background image into the bottom of the Layers Panel

You may need to use the Arrange Tool to fit your subject and background into the Image Canvas

If any of this is unclear, I've made a video of these exact same steps