In case you have purchased a TomTom device with lifetime maps. All the updates are free as long as your device is supported by the TomTom. You can directly go to the TomTom home download the updates without spending a single penny.
Apart from that, you can use third-party software to download free map updates on your device. Though these are not verified, you can use them to update your TomTom. One such tool is Map Share. Also, TomTom releases free map updates for every TomTom user.