Go to Play Store >> click the three horizontal line icon from top left(a drawer will open)[Or simply slide your thumb from left to right] >> Touch "Settings" from the menu that opens >> touch Auto-update apps.
Here, you will see three options from which you have to choose -
1) Do not auto-update apps
2) Auto-update apps at any time. Data Charges may apply.
3) Auto-update apps over Wi-Fi only