Often Android devices are set to automatically update and don't require users to update their apps themselves.
All you have to do to check whether your device is running the latest version of Checkout 51 is go to the Play Store and click the three dots top right corner. Then, select My Apps from the drop-down menu. If Checkout 51 is listed under the Updates section, click on it. If you don't see Checkout 51 under the Updates section, then you are up-to-date!