How do I regain interest in a class if I’ve lost confidence in the instructor’s depth of knowledge?

You've already paid for the course. That's what economists call a sunk cost. You won't get that money back.

However, your time is always worth something. The time in classes you've already attended is a sunk cost. Consider whether you want to spend any more of it to attend the remaining sessions.

Are you still learning anything from the course? Would you be able to get the same information somewhere else (such as by googling questions, watching online videos, etc.)? In other words, is there something that makes finishing the class worth your time?

If you do choose to complete the course, try to focus on your original goal when you signed up for it. If the instructor won't engage, perhaps you could use what's presented in the course as the starting point for what questions to investigate on your own. 

Best wishes!