how do i remove my icloud account from a device i no longer have access to?

iCloud: Remove your device from Find My iPhone - http://support.apple.com/kb/ph2702 - "If you’re no longer using an iOS device or Mac, you can remove it from Find My iPhone. When you remove a device, it’s removed from your Find My iPhone devices list, and if your device has iOS 7 or later, Activation Lock is also turned off. "

Remove an iOS device, Apple Watch, or Mac you no longer have

If you no longer have the iOS device, Apple Watch, or Mac because you gave it away or sold it, you need to remotely erase it before you can remove it. With Apple Watch, you must have watchOS 3 or later installed to remove Apple Watch.

Click All Devices, then select the device you want to erase. To erase:

• An iOS device or Apple Watch: Enter your Apple ID password or your family member’s Apple ID password. If you’re not using a trusted browser, answer your security questions or enter the verification code that is sent to your other devices. If you’re using your own computer and want to skip the verification step in the future, click Trust. If you’re using a friend’s computer, click Don’t Trust. If the device you’re erasing has iOS 8 or later, enter a phone number and message.
• A Mac: Enter your Apple ID password or your family member’s Apple ID password. If you’re not using a trusted browser, answer your security questions or enter the verification code that is sent to your other devices. If you’re using your own computer and want to skip the verification step in the future, click Trust. If you’re using a friend’s computer, click Don’t Trust. Enter a passcode to lock the Mac (you need to use the passcode to unlock it), then enter a message.

If the device is offline, the remote erase begins the next time it’s online. You receive an email when the device is erased.

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Do not forget to remove Activation Lock/Find My or else the new owner cannot use the device!

You would need access to the iPhone so you can sign out of iCloud.

What to do before you sell or give away your iPhone, iPad, or iPod touch - Apple Support