Can you change your mind after discarding during Monolith Arena?
I don't know this game at all but will answer this in a broader etiquette sense.
In games people make mistakes or they start taking a move and realise that they want to do something different.  When I play games with people the rule is people can take back things provided it has no effect on anything else or a random event hasn't happened.
So if a player (in any game) is asked to discard a card and then decides they should have discarded another this is fine.  No other player has taken a turn and the only person is disadvantage is the player who exposed a card.
If a player decides to take an action such drawing a card or rolling a dice then this couldn't be taken back after.
When playing games it's about trying to have fun and being reasonable.  Telling a player they can't undo something that has no effect on anything else would be unreasonable.