If you are an administrator for an organization (including your Sandbox organization), you can transfer any devices that you own to another organization of which you are a member. To do so, navigate to your organization Devices page and select one or more devices that you would like to transfer. After you've selected at least one device, click Transfer in the modal that appeared near the bottom of the page.
Once you click the Transfer button, another modal will appear with a dropdown menu containing all organizations that you belong to. Select the organization you would like to transfer the device to and click Transfer Device(s).
If you are an admin of the organization you are transferring the device to, the transfer will complete immediately. If you are not an admin of the organization you are transferring the device to the organization admins will receive a notification and will need to accept the transfer invitation.
Below is a video walk-through of the device transfer process: