Learn French: Il est en train de changer la robe parce que le mannequin est triste. - He is changing the dress because the model is sad.

In French, changer followed directly by a noun means to alter, modify, or replace that specific object. Here, a designer or stylist is physically altering the dress or replacing it on the model. If the sentence used changer de robe, it would mean the subject (he) is taking off a dress he is wearing and putting on a different one for himself!

Could changer take a direct object referring to the model instead of the dress?

Yes! If you want to specify who is getting dressed, you use changer quelqu'un. For example, Il change le mannequin would mean he is changing the model's clothes. In our sentence, la robe is the direct object, so the focus is entirely on the dress itself being altered or swapped out.

Why is it le mannequin (masculine) if the model is likely a woman wearing a dress?

The word le mannequin is always masculine in French, regardless of the gender of the actual person modeling. Even if you are talking about a female fashion model, you still use the masculine article le and make any adjectives agree with the masculine noun.

Could I just say Il change la robe instead of using est en train de?

Yes, you absolutely can. The simple present Il change can mean he changes or he is changing. Using the expression être en train de followed by an infinitive just strongly emphasizes the continuous, ongoing nature of the action right at this very moment, telling us he is currently in the middle of doing it.