Option 1 - Adding a background colour

Step 1:

Open a design or create a blank canvas. Click on the colour palette in the panel found on the right of your screen titled background colour ↓

Step 2:

You can also add a gradient effect to your background by clicking on the gradient toggle found in the colour palette. Click on either of the buttons on the slider below the gradient toggle to add the two colours of your preference ↓

Step 1:

From the left corner of the Editor, the tool go to the elements panel, then click on backgrounds ↓

Step 2:

You can narrow your search down by typing the keywords of what you are looking for in the search bar below.

Simply click on a background image of your choice and voilà!

Step 3:

You can make adjustments to the overall appearance of the image by clicking the Edit image

Step 1:
Open a design or create a blank canvas. Click add image in the panel found on the right of your screen titled background image ↓

Step 2;
Once you have added your image, you can adjust the size and position according to your preference by clicking on reposition ↓

Step 3:

You can also make adjustments to the overall appearance of the image by clicking on Effects ↓

Step 4:
In order to remove or replace the image you just added, simply hover your cursor over the thumbnail of the background image and click on the Remove or Replace button that appears ↓