As you're probably aware, both exams are 50 minutes long with 80 questions on each paper. Whilst they are both split into sections very similar to the equivalent VR & NVR NFER/GL Assessment test papers each section is not timed individually so it is really important that you teach your child how to manage their time. I did this by first trying to get by DS to work towards a target time per question - to be exact it works out at 37.5 seconds per question so my advice would be to try and strive to get down to around 30 seconds per question. This way, if your child gets stuck on a question for a little more time either because it is a little more tricky or is one of the longer VR types then overall it should even out.
Once his speed was reasonably good I then got him to break the test time into 10 minute chunks - i.e. I got him to look at his watch every once in a while and when he saw that 10 minutes was up he would know that he needed to be on or around question 16, when 20 minutes is up question 32 etc. This approach works regardless of how long the test paper is. This ensures that your DS does not get bogged down on a single question and burn all their time up. Instead, it keeps them focused on moving forward. We also found that some test papers were a little more generous on timings - however, my DS still tried to keep to the pace he knew he would need to be at for the Tiffin exam.
In terms of changing answers, you do get a pencil & rubber on the day so you can rub out your answer and mark another box. Also, get your DC to practice doing this before the test itself as they need to be able to make sure that the remarked answer sheet is machine readable - i.e. what they might think is machine readable might be different to what your and I might think would be readable