What is the effect of damaging a creature that already has damage marked on it?

When a creature has damage marked on it, and you deal additional damage to it, the total amount of damage marked on the creature is what determines whether it can stay on the battlefield or if it will be destroyed. If the total damage marked on the creature equals or exceeds its toughness, the creature is considered to have received lethal damage and will be destroyed as a state-based action.

This reflects the fundamental rule that a creature with damage equal to or exceeding its toughness will be sent to the graveyard due to having lethal damage. The additional damage does not affect the ability of the creature to attack or block, nor does it lead to its exile. Additionally, while regeneration can be a potential option when a creature is about to be destroyed, it is not directly related to the question of how existing damage interacts with new damage. The critical aspect here is understanding that the cumulative damage is what can lead to the destruction of the creature.