If the company modify the employment contract, can I refuse to sign the amendment if I don't agree with it?

If you do this there is a good chance you will loose your job and it will follow you that you did this . good luck .

You can modify the contract too.
Maybe all amendments to the original could result in your salary being doubled.

One important feature of a contract is the intention on the part of the two parties to create legal obligation. Essentially, a contract is an agreement. You can choose to reject the modification of the employment contract, thereby rendering it null and void. You can contact your lawyer for more advice.

since most employee are at will, you can refuse to sign the new policies and procedures of the company, and they have the right to terminate your employment with them for not signing the new agreement

You can; however, the employer can also refuse to offer you continued employment if you fail to sign.

Most regular employment is "at will" and not subject to contracts. As for policies and procedure manuals, you don't have to agree or disagree - you sign to acknowledge that you have been given a copy of and are aware of the current policies and procedures. If there are terms that you do not want to be subject to, then you have the option to find another job.

If, however, you do have an individual contract with the company, accepting the contract is usually determinative of your continued employment and so you usually have the ability to counter-offer and bargain for your employment terms.

If you are part of a collective bargaining agreement (union), then as a member of that union, you agree to whatever terms your union accepts.

As long as the employment contract wasn't modified during the duration of the contract (ex.: One year contract and modified less than that years end) they cannot legally modify it and would be in breach of contract. However, if your contract term has ended they can modify/change to whatever they see fit. You can refuse to sign as everything is negotiable, but you may also take a chance they will not re-employ you.