Make sure you have both devices linked to your account at the same time (see add a GPS if you need help) then follow below steps.
1. Select the new GPS where you want to have the subscription moved to.
2. Tap the gear in the top right corner to access the settings for this GPS.
3. Select subscription in the settings.
4. Select Transfer subscription from another Cube.
5. Select the Cube that you will be moving the subscription from to complete the transfer.
Once the transfer is complete you can go back to the device list by clicking the back arrow in the top right corner twice.
Now you can swipe left on the old GPS if you want to remove it from your account.