How do I change my icloud account?

Hello. This is AethereAl asking for audience on how to remove the iCloud account I had on my phone right now. It was the account of the past user of the said phone and the owner/user said he can't remember the password on his iCloud. He's Korean as well so I don't really understand their language if I tried hacking it to remove it. Then just now, I'm suffering on how to open up my iPhone for I don't remember my passcode. It was kept away from me so I can focus on my studies but now it's back, I don't remember anything about my passcode. So I tried looking on to Google. There are three ways. I already synced this on a laptop. But it needs my passcode which is my freaking problem. Next would be the recovery mode. But I don't know if this works and I'm currently doing it. The last is the syncing of iCloud account. It's not mine, the iCloud account, so the third method is useless. I'm hoping you can help me on my problem since it's a discussion from the major site of Apple. I love the features on this phone so please don't fail my expectations. I'm starting to think Android is better. Just please help T_T