Tensioning an up-and-over garage door spring should only be done by a trained professional due to the high risk of injury. Adjusting the tension involves manipulating the tightly wound spring, which can release with extreme force if mishandled. If you suspect your garage door spring needs adjustment, it's crucial to hire an experienced technician who can safely assess and adjust the tension to ensure proper door balance and smooth operation. Attempting to do it yourself can be dangerous and is not recommended unless you have the necessary expertise and tools. Prioritize safety and consult a professional for this task.