Question: If I discard a tile but change my mind and want to discard another tiles, can I take it back?

Answer: Once a tile has been fully named or has been placed on the table, it cannot be taken back.  As the National Mah Jongg Leagues says, “Down is Down.”

Source:  Mah Jongg Made Easy 2020 page 19 #7 , page 21, #18, page 23 #5