It’s common to receive a mail from some service that they’re changing the terms and conditions of their service (internet provider, gmail, etc.) These changes are unilateral and can be about anything, from their privacy policy to the money they intend to charge me.
Why can’t I unilaterally send them a mail and say, I am changing the terms of my patronage and will now insert my new policy (pay only xx amount, or demand a mint Chico chip ice cream every Tuesday)?
Unfortunately (or fortunately, probably) contract law doesn’t really work that way. If you sneakily change a contract or even verbally misrepresent what’s in that contract, it can be challenged in court.
It can be difficult to prove though. For example, if a car sales person verbally says your car payment will be $100 but the contract you sign says $1,000 then you can argue in court that they misrepresented what you signed and the $100 should stand. But it’s hard to prove what was said was different than what was signed.
Same concept if your landlord signed something, sent it to you to sign and you modified it to something else without communicating a change. If they can show the original document they sent you, you’re boned.