It means they are taking the dare further

Thank you, Gary.

Shannon is right. 'upping' is slang for 'raising' - and means 'raising the stakes' here.

I'm not sure you could use "upping" if you were playing a game of Scrabble. While Urban Dictionary has a few very dubious suggestions, upping is really only associated with gambling. When playing poker someone could "up the ante" or be "upping the ante" by making higher bids. Sometimes business can be a gamble so someone could be "upping the stakes" by the increasing the amount someone is set to lose if things don't go well. In the sentence "Are you upping my dare for me?" I think "for me" is redundant and I would change "my" to "the". "Are you upping the dare" could therefore possibly mean to increase the level of a dare someone else needs to do by "upping" what you are willing to do, but I have never heard this said before, but then again I don't play Games of Dare https://en.wikipedia.org/wiki/Games_of_dare Perhaps someone is dared to do something, and then the person changes their mind and challenges them to a bigger or more dangerous dare. Maybe then someone might say "Are you upping the dare?