HR made me sign 2nd contract for lower salary

I am a fresh grad and I have been working for this company for a month now. I don't like the environment and the people but I still continue because I don't wanna quit immediately. However, I am now induced to quit because of something. The HR manager made me sign another contract (same content, aside from the salary). They are now offering me a salary lower than the basic pay that they've given me initially. Their reason is that the HR mistakenly offered me a higher salary than the usual offer for fresh grad probi employees. My bad because I signed the contract because I felt pressured by the HR manager. After that I felt devastated because I wasn't able to stand for my right and I feel downgraded by it. Now I am sure I wanna quit the company. Question is, do you think I could still do something about what happened even if I already signed the 2nd contract? Also, I really want to leave the company asap because of the reasons that had already multipled. What do you think is the quickest, legal way to terminate my employment? I really need help. Thanks

render 30 days notice

No need to sign it.
Non diminutions of benefits ang magging problema ng employer mo.