To prepare for a Machine Learning interview, you need to start by refreshing your understanding of the basic concepts of math that are needed for Machine Learning, such as linear algebra, calculus, statistics, and more. Then, you can start practicing common interview questions, such as the difference between overfitting and underfitting and how to overcome both, etc.
If you are looking for an online course to learn Machine Learning, I recommend this Machine Learning Online Course by Intellipaat.