I bought 2 Thunder tumbler radio control rally cars. They don't come with instructions for how to change the frequency so they can race. Any idea how to do this?

The frequency cannot be changed in the cars. The frequency is set, you have to buy two different frequency cars. There are stickers on the cars, and there is also a label on the outside of the box. The frequencies are 27 and 49, we got two 49s, so we ll have to exchange one.

You can't change their frequency, you have to but a car with them designated, black ones a 47 and red/blue are there 26 or whaterver.

They are running the same radio frequency signal!

take out the batteries of the other car and don´t put them back in

Most of the store bought RC cars support a few channels and autoselect the channel or they are tied to the controller. Some of the better (hobbist) RC cars have a channel select on the underneath. You could check for that but if these are the 9.99 stocking stuffer specials, you may be out of luck.

How to Chang the frequencies on the thunder tumbler

The box shouldn't say "two frequencies for competitive racing." That is misleading.

MAYBE YOU SHOULD TRY TAKING IT BACK TO THE STORE AND TALKING TO THEM ABOUT IT OR YOU COULD BUY A NEW ONE AT AMAZON .COM FOR $11.95
HERE IS THE LINK FOR MORE DETAIL:
http://toys.wslrss.com/?s=THUNDER+TUMBLER

Find out who owns or pays for the website and you'll have a contact number. There are sites that'll give you that information usually for free.

tried that said they would send me one , brandon was the reps name, 2 weeks now nothing,