You can add multiple devices per entrance and location. If you want to move a device from entrance to another or just to remove it, you can do this in the essentry Dashboard by unpairing that device.
First, you have to find out the name of the device and of the entrance and location. All this information can be found on the device information page as described in this instruction: How can I get information about a device?
Next, open the Dashboard and select the location and entrance of that device. In the detailed view of the entrance, you can see all the devices currently paired with the entrance. To unpair a device, click the red "x" next to the device.
Once you have confirmed the unpairing, the device will be removed from the entrance and can be placed at another entrance if needed. For this, the device will display a new pairing code that can be used to pair it: How can I connect a device to an entrance?