If I saw that someone is cheating in his speedrun how can I report him?

You bring it up to the moderators of that board.

but... How? Where's button ?

There is no button, you contact them via social media, if they have any, or through the game leaderboard's forum

K thx

another question cuz I don't want to create one more thread how can I become game moderator if it's possible?

If the games moderators are active, you can ask for it. Don't expect it just because you ask though. If they aren't active (i.e. haven't logged in / verified a run for 3 weeks), you can go to the moderation request thread and ask there.