I signed this at the beginning of my lease and now they are trying to charge me an extra 100$ a month for parking. Can they change it?

I am an attorney. There are a lot of bad responses here.

Your landlord is obligated to the terms of the lease agreement. That means they cannot unilaterally (all on their own) change the lease terms. So, they cannot decide to charge you $100/month extra for parking. I would send them a copy of this agreement and tell them that you refuse to pay any additional parking fee.

PS- You will need to move at the end of your lease. Start looking.