To replace the battery in your Veterinary Heart Monitor, remove the monitor from your iPhone. Push the iPhone through the camera cutout, while peeling back the monitor from this corner. The battery compartment is located on the inside of the monitor. Use a 1.6mm Phillips screwdriver to open this compartment and replace the dead battery with a new coin cell battery, 3.0V. Be sure to use the same battery model type. These batteries can be found at most general-purpose stores.
Make sure that the protective sticker is removed from the battery (if applicable) and that the battery is inserted with the proper polarity (you should be able to read the writing on the battery after placing it into the compartment).