To add single tiles into a tiled section, click 'Single Tile' in the Editing menu.

This will load up a menu with tiling options for you to select from that are similar to the already chosen tile.

Once the tile is selected, hover your mouse over the tiled section and this will display a blue rectangle (that moves with your mouse movement) showing you where you can place the single tile. To lay the single tile, left click on the tile you wish to change.

If you make a mistake, click the tile you have changed and it will revert back to the original.