You can easily change the device that is being used to record an EKG. To do this select Record Your EKG, then select the Gear icon in the top right hand corner. Next select change device, and select the device you would like to use to record.
You can also switch your device in the Profile Tab. After selecting the Profile tab, select Manage Devices, then "EKG". From this screen you will be able to change the device used when recording EKGs.