How do i update a apk app

Through the app store.

It will let you know if an update is available. Also you can go to the play store and look at your installed apps. If there is an update, it will say so.

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Or, if you didn't download the app from Google Play, then go to the app store from where you got it by opening up that store's app. Most of them are similar to Google Play, in that they will show you available updates in their "your apps" section.

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or if you installed apk by downloading it from website. do the same with the updated apk file by downloading and installing it.