In order to add the existing odometer value (the driven kilometers/miles) of your vehicle(s), the first thing you have to do is open the settings of your vehicles (check here how).
Once you open the main menu, at the bottom of the panel you will see the value Odometers, as it is shown in the image below.

From there you can choose if you want the values to be counted from the GPS Tracker of your vehicle or from a sensor. If you choose the option GPS, all you have to do is write the current value that is on your vehicle and press the save button.
If you choose the option Sensor, you will need to add a new sensor about it (check here how to add sensors) and on the type of the sensor choose the option Odometer as it is shown in the image below.
