Will my users need to re-download my app after I update it?
No. The updates to your app are handled automatically by Apple and Google.
How long will it take for my update to reach my users?
This is different for Apple and Google.
With Apple the update first has to go through review, this can take between 5 and 10 working days. After that it can take upto 12 hours for the update to get released throughout the App Store.
With Google the update takes about 1 day to move out to all devices.
My update has been processed but I haven’t gotten it yet?
Updates can be delayed if your device is offline, has low space, low battery and a host of other variables. Apple and Google both, stage rollouts of app updates to prevent too much load on a server. Your device will normally not download an update until a few are available to conserve battery.