If the device you have is a production device, then this cannot be done and you will need to purchase a dev kit for testing your app on device.
However, if the device you have is a dev kit (Sandbox Device), then you would need to simply disassociate the device from the current test merchant and then associate it to your test merchant.
To disassociate a device you must:
1. Log into your developer account (developer who's test merchant is associated with the device)
2. Go to the Dev Kit tab of your Developer Dashboard
3. Scroll down to the device you would like to disassociate
4. Click disassociate
(Make sure the device is online when disassociating. The device will be alerted by the server to factory reset and disassociate the merchant.)