Apps update themselves automatically, but you can choose for them to update only when you are in Wi-Fi mode to avoid unexpected costs.
Be careful, when your Wi-Fi signal is weak, your mobile could be connected automatically to 3G or 4G and you maybe charged for internet without knowing. If you dont want to be charged from mobile data, deactivate Wi-Fi assist.
Go to settings on Plays Store, then auto-update apps and select "over Wi-Fi only".
Go to settings, then "iTunes store and App Store" and de-select "update".