My uncle recently gave me a used iPhone 6?

My uncle recently gave me a used iPhone 6.Everything is fine except that I don't know the past users iCloud password. I need to know if I erase All settings and content would that delete the old users iCloud info.so i can put my info into it

Since iCloud is a security measure, there's no way to just remove it like that, the only way to do it requires the owner's login details. Every reset method will take you back to that point, needing the previous owner to login.
Your uncle is going to have to proactive and look into ways to recover his password, that's the only thing that can be done.

It probley would, see if you can back it all up first

If find my iPhone was never turned on then you can easily reset it. If it was turned on then he will have to reset his iCloud password.

Follow these steps:
Back up your device.
Tap Settings > iCloud. Scroll down and tap Sign Out. In iOS 7 or earlier, tap Delete Account.
Tap Sign Out again, then tap Delete from My iPhone and enter your password.
Go back to Settings and tap General > Reset > Erase All Content and Settings*. In iOS 7 or later with Find My iPhone turned on, you'll need your Apple ID and password.
If asked for your device passcode or Restrictions passcode, enter it. Then, tap Erase [device].
Contact your carrier for guidance on transferring service to a new owner. If you aren't using a SIM card with your device, you can contact them to get help transferring service to the new owner.

No, it won't. The only way to fully reset and activate an iPhone that is locked to an Apple ID is to know the password to the Apple ID. That's the drawback to stealing a phone instead of buying one.

You won't be out anything. Try erasing it.