To remove the background one color at a time:

In the toolbar, choose Instant Alpha from the Select pop-up menu.

To remove the background, drag over part of the background. Preview removes the area you dragged over and any pixels adjacent to it that have the same colors.

To restore part of what you removed, hold down the Option key as you drag over what you want to restore. Preview adds the area you dragged over and any pixels adjacent to it that have the same colors.

Press Return.