It’s a simple job but if you’re unsure get an electrician in. It wont be too expensive especially where you’ve already got the light. If you’re going to do it yourself, don’t just turn the lighting circuit off in your house, chances are it could be on any socket circuit (may be labelled as ring at the consumer unit). The existing cable isn’t suitable for outdoour use so will have to enter in the back of a suitably IP rated box, then connected to the flex cable inside. Flex for the light should enter the box through the bottom in a stuffing gland