Applications don’t need to be uninstalled before they are updated. If you are side loading apps (installing apks without using the Play Store), you can simply install a newer version of a previously installed app and it will be over written.
However, if you are looking to update officially from the Play Store, you can easily do that by running the Play Store app from your device. Press the menu key while using the Play Store app and select ‘My Apps’ from the list.
The new page will list all the applications you have installed on your device and their states. While some applications can be automatically updated, others need to done manually. The list will be divided between ‘Updates’ and ‘Up to date’. The Apps listed under ‘Updates’ can be updated one by one or all at once by tapping the ‘Update (number of apps)’ button on the left.
Before updating an app, you can tap it to open its main page and read the ‘What’s New’ section to see changes over the last version. Sometimes an app’s new version may do away with features you want; in such a case, you may not want to update the app.
However, if you wish to proceed with an update, you can tap the ‘Update’ button next to the ‘Uninstall’ button on the app’s main page.
After your apps are updated, the notification panel should show you successful results as confirmation.