As soon as you can, move to a different part of the carriage. If you are frightened, TELL someone immediately. Choose a family group rather than a single person, and ask if you can stay with them until you feel safe. They are marked out in red. If the nearest one is too high, look for another one nearby which you can reach by climbing on a seat.
Tubes have pull down handles and push buttons.
Some BR trains have emergency chains to pull rather than handles. They all have the same effect. If the platform is empty or there are people on it who make you uncomfortable, you can usually stay near the ticket office, or the ticket collector, until the train appears. If someone on the platform makes you anxious, go to wherever there are people around who make you feel safe. It's better to miss a tube and get the next one than put yourself at risk.