Can anyone tell me where do I exchange the event coins?

Many people tell me to just go to the middle of Detlas, but I still do not see where it is.

 

Depends on which coins you're trying to exchange. The main token exchange merchant is in the carnival area of the festival where the bottle shooting minigame and other token merchants are. Here, you can turn talespun memories into gold tokens, or convert silvers/golds into golds/diamonds and vice versa.

There's also a token conversion option for talespun memories in the main minigame lobby from the trinket merchant there.

Keep in mind that there's no way to purchase tokens with emeralds (for some reason), so if you're looking for that option you're out of luck.