The target can reroll any of the damage dice. They must accept the rerolled result.
The target gains a bonus to the damage roll equal to your Charisma modifier (double your Charisma modifier at 5th level; triple it at 8th level).
You can spend an additional command point (1 max) to add another damage die of the same type to the damage roll when you reroll damage dice. (You only roll the extra damage die once.)
You can also use this command as a free action (instead of as an interrupt action) when an ally hits with an opportunity attack.