How to make a character float up in a 2D game

I want to make a 2D platformer where the character moves up instead of forward. The character gains height the more balloons they collect. I have instructions for making a standard 2D platformer with forward motion, jumping, idle, coin collection, moving platforms and enemies. Now I want to modify it to take the player up and switch coins for balloons and fruit (the fruit give the player extra lives).

The only issue is I'm not sure how to make the balloons affect player gravity. I am very new to game making, t$$anonymous$$s is for a class. I want to challenge myself, but will sacrifice the floating if it proves too difficult to accomplish by Jun 18th (the project due date).