Can you watch lessons multiple times?
YES! You can watch, rewind, pause and come back and watch again. There is no limit to how many lessons you can take and how many times you watch them.
The only limit is that you must "own" the product. So if its NBT or AP Maths then you have 1 years access from purchase date. If It's Maths Online then you need to have an active subscription.
When you complete a lesson can you go back to it?
YES! Marking a lesson as complete helps you to track what you have learnt. It doesn't mean you cannot go back to it in the future.