We have played only 4 times so far. On a meld with a joker or 2 in it that has been laid down, is it allowable to exchange the two or joker for a card of matching numerical value ( say 8) if you pick it up? Even on an incomplete meld, can this be done? Thanks. I know it is a silly question, but we are SO new to the game.
No. Once a meld is laid down, you can add to it, but you can't ever take cards back away from it.
There is not really a specific rule I can quote saying this; rather it is simply that none of the rules allow you to do it. It's kind of like asking if you can steal a card from your opponent's hand... not mentioned in the rules; simply not part of what you're allowed to do.