You can simply reassure the completion of your targets and respond with a promise of an enhanced show in the upcoming evaluation
You can simply apologise and ensure that repetition of the same would not happen in future
Your tone has to be apologetic in this case too and again, show them you have learnt your lesson!
Sometimes the accusation or reason stated in the warning letter is untrue or deniable. Then, you have to just politely decline the letter acceptance and challenge the same for a reconsideration
You can take a step back if the situation gets really bad. It is better to leave the organization yourself with grace than to be waiting for the termination letter.
Thus, do not frown or get frightened the moment you receive a warning letter. You can download and go through the different warning letter format and templates available on HROne for understanding the cases better and respond wisely.