Select the item you wish to adjust.

Four icons will appear at the bottom and center of the canvas. The third one, two circles and lines, is the image editing icon. Click this button.

You will see icons for rotate, brightness, blur, contrast, opacity, saturation, and hue. Once you click on any of these settings, a slider underneath enables you to adjust the corresponding image settings. You can click and drag the dot to make adjustments, or you can just click on any point of the line. The category you're editing will display the degree of change you have made to that setting.

At any point, if you want to revert to original image settings, click the "Reset All" button on the left.

Once you've finished editing the item image settings, click the blue check mark on the right to save the altered image settings, or click on any blank area of the canvas or press the esc button on your keyboard to deselect the item.