Log out of Apple ID

Erase a device:

1. Open the Find My app and choose the Devices tab.
2. Select the device you want to remotely erase.
3. Scroll down and choose Erase This Device.
4. Select Erase This [device].

When you erase your device, all of your information (including credit, debit, or prepaid cards for Apple Pay) is deleted from the device.

Also click Remove from account.

From the article below.

If you forgot to turn off Find My before you sold or gave away your iPhone, iPad, iPod touch, Mac, or Apple Watch, you can turn off Activation Lock using Find My iPhone on iCloud.com.

If you still have the device, see the Apple Support article Activation Lock for iPhone, iPad, and iPod touch, Activation Lock for Mac, or About Activation Lock on your Apple Watch.

1. In Find My iPhone on iCloud.com, click All Devices, then select the device you want to erase.
2. If you don’t see All Devices, it’s because you’ve already selected a device. Click the name of the current device in the center of the toolbar to access the Devices list, then select a new device.
3. Erase the device.
4. Because the device isn’t lost, don’t enter a phone number or message.
5. If the device is offline, the remote erase begins the next time it’s online. You receive an email when the device is erased.
6. When the device is erased, click Remove from Account.
7. 
8. All your content is erased, and someone else can now activate the device.

Find My Phone – Remove device.  -