WorldEdit allows editing of the terrain etc.

//wand
to get a "wooden axe" wand. Left-tap the first block in the selection, right-tap the opposite block in the selection.

//replace dirt
to replace the selection with dirt. Use other ID numbers or names to fill with other blocks

//set 3
to set the selection full of dirt. Use other ID numbers or names to fill with other blocks

MORE INFO
Beginner's Guide
ID numbers and names