All installed apps will get the new version of your app once you publish it.
Once an app is installed on a device it has information cached on the device. After you make changes and press the publish button, it can take up to 10 minutes for the device to recognize that changes to the app have been made and automatically update it. If you publish the app with the no cache publish option, the changes will take immediate effect since the images are not cached on the device.
If you make changes to the icon image, that isn't updated on android devices. To get that updated on a device, the app needs to be deleted and then installed again.
On rare occasion, apps get stuck and do not update. Turn off caching and publish to solve that issue.
If you delete an app, all installed apps will stop working.