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)?
The contracts almost always have a clause that says they can change the terms and conditions at any time. There is no such clause saying you can do the same, but you probably could; they just wouldn’t agree to it.
Tho, depending on what you’re talking about: It might not be a real contract (see: terms of service agreements for software where you never sign anything).