Thank you for your question. Without knowing how the leaves were torn, I can't comment too much, except to say that if you find them to be unsightly, you can remove them. As to the other problems, the following Extension article gives you some ideas about the cause and cure of the other issues:  https://gardeningsolutions.ifas.ufl.edu/plants/houseplants/dieffenbachia.html

Hope this is helpful.  Good luck!

Thank you!! Also, the leaves were torn by people bumping into them/my brother’s friend hits them purposely. Where would I cut off the leaf? Just anywhere? 

If the wound has healed over, and is not weeping fluid, you can just leave it. If it is unsightly and you want to 'clean up' the plant, cut it as close to the origin of the leaf as possible. If you have a cat or a dog, their eating any part of the plant is toxic to the animal:  https://www.aspca.org/pet-care/animal-poison-control/toxic-and-non-toxic-plants/dumbcane  You might want to move it to a more protected place, or put a fence around it.   Or put a cage around your brother's friend.

Hahah thank you so much! You have no idea how much you’ve helped my case

Thumbs up!