If an icon you placed can be resized, you'll see a blue box around the icon after you touch, hold and release it. Drag the edges of the box to resize it.

Touch and hold the icon you want to customize, then release the icon. An editing icon appears at the upper-right corner of the app icon.

Tap the app icon (while the edit icon is still displayed).

Tap the icon design you want from the available icon choices, then tap OK.

OR

Tap the Add new icon (at the top of the screen), set the size for the icon, then tap OK. Select the image you want to use for the app, crop the image (if necessary), then tap OK.