It depends. First, I will ask my manager if I can ask for a leave of absence and attend the training another day. If the manager tells me that it can be delayed, I will email the customer and tell them I'm available at 10 o'clock. And then I will attend the training another day.
If the training is too important to miss, I will email the customer sincerely telling them that I'm really sorry, but I have something really important at hand,
I will definitely reply to you once I've finished, and ask for forgiveness and understanding, and apologize again.