how can i do this?

i want that when the player collision on the second platform run… if i touch again return to the first platform

Not really sure what you mean,
can you draw an image how it should work or give more details?

So you want the player (circle) to jump through the top platform but then stay on it, then it starts to “run” in a certain direction. If you press another button it falls through onto the bottom platform and stops “running”?

this

You can look at flipping the gravity for that object i guess. Can I make gravity work the other way round for one object? - Questions & Answers - Unity Discussions

but this webpage help me to do it?

but i need the code in C#

No one is going to write code for you…

look at the docs: Unity - Scripting API: Physics.gravity

you can set the gravity to what you want.

So it could be -9 then when you want to flip you can set it to positive 9 etc…

but i dont want it…

i want when the player touch the second platform can run… and if i touch again jump to the first