How to unlock my Iphone if I forget the password?

I used my iPhone 6 until its battery died. I didn't replace its battery for almost 3 years now. I got it replaced yesterday and I tried several passcodes but couldn't identify the correct one. I changed my iCloud account when I bought a new iPhone. So I don't have any sim card, iCloud account, or passcodes for this iPhone 6. Trying to "Find my Phone" is also useless.

In these circumstances, does anybody know, what could be a proper solution to open the iPhone?