How to logout from iCloud without password? and without resetting I have an iPhone 6 Plus, and the previous owner forgot to sign out his iCloud account.
What should I do? Can I reset this? It may upgrade into the latest ios which will make the phone crash, and I am afraid it might downgrade to iOS 9.
You cannot sign out of Find my iPhone without the password from the previous account.
The previous owner needs to sign out from the web, the device does not need to be returned to complete this process.
These are the instructions the previous owner should follow. The new owner is you.

If you no longer have your iPhone, iPad, or iPod touch
If the steps above weren't completed and you no longer have your
device, use these steps:

Ask the new owner to erase all content and settings using the above steps.
If you're using iCloud and Find My [device] on the device, sign in to iCloud.com/find or the Find My app on another device, select
the device, and click Erase. After the device has been erased, click
Remove from Account.
If you can't follow either of the above steps, change your Apple ID password. This won't remove personal information that's stored
on your old device, but it prevents the new owner from deleting your
information from iCloud.
If you're switching to a non-Apple phone, deregister iMessage.
If you're using Apple Pay, you can remove your credit or debit cards at iCloud.com. Choose Account Settings to see which devices
are using Apple Pay, then click the device. Next to Apple Pay, click
Remove.
Remove your old device from your list of trusted devices.


https://support.apple.com/en-us/HT201351