You can quit an active Classic game from the match list on the main screen. To do so, press and hold the game for a few seconds and then select Give up.
If the game is on hold, you can open it and select Give up.
Keep in mind that this will negatively affect your statistics since the game will be considered as lost. That is why, if you receive a request for a game in which you would not like to participate, we recommend that you reject it before it begins. This way, your statistics will not be impacted.