Is there any way or power-up to quickly lose weight or update this content in the future? I want to go back to my previous weight or repeat the gains
Currently, once you move on to a new weight stage you cannot go back to a previous one. That will be true of the game always since that is the goal of the main character. However, in the next update I am adding a gym which can be used to help you lose weight so that you can stay at a certain weight stage for longer if you so choose, along with other things do there, but I don't want to spoil too much.
I actually really enjoy your game; however, I am slightly disappointed that your character cannot revert to a lower weight stage, especially when her weight can go down. For these types of games, I love being able to make my character chubby, make her thin again, then repeat the process in a nice loop. It's sort of funny that she can claim, "Looks like I weigh 100 pounds. It's official, I'm fat now!" Regardless, the addition of a gym is an excellent decision.