The attacker does not have the opportunity to retreat if the attacker wins the combat. The only allowance for retreating is for the losing army:
After suffering casualties, the losing army must retreat from the embattled area (supporting units do not retreat.)
There is no mention of what happens to the winning units after combat because nothing needs to happen to them. Combat began by them moving to the territory they are trying to conquer. Thus, all that needs to happen in cleanup following an attacker victory is the removal of order tokens:
After combat is completed, remove the attacking player’s March
Order token from the game board.
If the combat was won by the attacker, remove any Order token the
defender had assigned to the embattled area (if one remains) as well
as any Power token in the area (which would exist if the defending
player had previously established control there, see page 24)
One pseudo-exception to this is Loras Tyrell, who moves the march order with the attacking units if they are victorious. You can use Loras to move attacking units back to their original territory after they win a battle, but doing so is not a retreat (these units will remain upright) and is not immediate (this cannot be done before your next turn in order resolution).
