You can't change it. You agreed to a price that was in the contract. If you feel you deserve more, talk to your client and discuss it with him. He/she can add another milestone and provide you with extra payment.
The short answer is NO.
It is unethical, unprofessional and a breach of contract.
However it depends on the contract you and your client established. I suspect most contractors (and clients) here don't create anything more than what's written in the job description or vaguely discussed in the interview process.
Experience will teach you to anticipate potential problems, scope creep, and probable revisions by the client, so that you can prepare a contract that will allow you to do the complete task at the rate you and your client initially agree to.
If on your current project(or future milestones), you and your client can agree an increase in the fee because of certain cercomstances than an ammendment to you contract would be fine but only if both of you agree to it.
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