My website uses cookies and I've noticed that I'm able to change the value of the cookies with a chrome plugin. Is there a way to make it so people cannot edit the cookie? I've been thinking of writing a script to check if the cookie changes but I'm not sure if this would work. Any help would be greatly appreciated, thanks.
No, cookies are stored on the client side and you don't have any control over them. You have to validate them on the server prior use every single time. When it comes to web development, you need to see your clients as a potential security liability. You can't trust them.
If you want to make it a little harder for the attacker, you can encrypt contents of your cookies, but the fact they're on the client side means the attacker can try to decrypt them and modify as wishes...
=> If you have data you need to keep away from users, don't use cookies for that. You should use Session in that case.