The method that I use here has always worked well for me, but I do not take responsibility for my own attempts (especially at Official Match Balls)!
• replacement valve
• Ball pump
• Repair kit
Before you think about changing the valve, you should check if the valve is broken. Instead, the ball could have a hole or you use too thin a ball needle when inflating.
I hope I could help you. If you want, leave me feedback on the contact page, as you can tell with the instructions.