Ask for a reconsideration and send them a report detailing why you think their decision is wrong.
Pick each category on their assessment apart by explaining why you think they are wrong and always describe yourself on your worst day.
If the reconsideration doesnt work then you request a tribunal.
Dont forget though the mobility component of DLA is different to PIP. When PIP was introduced they made it harder to get as if you can walk over 20m even with walking aid then you dont qualify.
If you get to the tribunal level you will need advice and there is a few places you can go.
There is a very high success rate for appeals.