What should I do as a TA if the instructor is not teaching properly?
I'd bet that the grad student teaching the course knows very well that his or her teaching competences are lacking, and that the students struggle to follow. After all, even if this person has little experience teaching, he or she has a lot of experience being a student, and is thus likely to recognize poor teaching, but unable to do something about it.

But alas, such is life sometimes. The grad student cannot suddenly, by magic, become a teacher with 20+ years of experience. Nor should they be expected to. The department, who already made the decision of putting an untried grad student in charge of a course, is also unlikely to suddenly allocate more resources.

It would be perfectly fine if you did nothing. Nothing is expected of you, you will not be paid for doing more than you are asked, and if you just do it, no one will thank you.

If you feel compelled to do something anyway, here's a suggestion. Go to the grad student and say that it is your impression that the students struggle way more with the material than last time you TAed the course, and ask if there is anything you can do. Prepare a couple of suggestions, stuff that you know could be improved by going over it a couple of times more in lectures. Don't pass any blame, but try to be as constructive as possible.