Will updating an app with a sideloaded apk delete app data? I have an application and I want to update it . I have downloaded latest version apk and I would like to know if i sideload the latest apk would I loose old application data .
No. If you don't uninstall the existing app, and you use adb install to install the new APK, the existing app data will be preserved.

Note that you can only do this if the package names match and the old and new APKs are signed with the same developer certificate:


If the package names don't match, then both versions will be installed side-by-side, and the new one won't have access to the old one's data. This might happen if the developer has slightly different variants for the app depending on the distribution channel (e.g. a variant for Google Play, and another for direct APK download).
If the signatures don't match, then installation will fail with an error message telling you that's what went wrong. The only way to fix that is to use an APK signed by the original developer.