Preparation for PM interviews

Just had the first round for a PM interview (had already done the initial interaction), was given an assignment which we discussed. Then the person told me they'd be taking 3 case questions right at that moment - Completely unprepared.

Went okay I guess and they told me that next week I'd have another round with 2 of their teammates.

Pls help a bro out with what all should I prepare for the next round. :)