My 12 years old bro has down syndromme and can't. Talk like a norm person. What can I do to help him. He mumbles and smart in playing gadget.?

You're helping now! Asking questions is the most important first step. Good job! kids with down syndrome speak and play differently sometimes because that's just the way they are. But they also have special ways of being the best brother ever, and they need their families to help them. Being patient and just being happy with him, even if the gadgets are hard for him to use, is a really big help.
Speech therapy. He may need to see a speech therapist; meanwhile you may stimulate his speech ability by always talking and reading to him and have him repeat some of the words from your reading.