The device, I am using for getting Windows Phone 8.1 developers preview update is Lumia 525 and in case you are not familiar with what is developers preview update here is our step by step guide to updating your device to Windows Phone 8.1 much before final public release.
Coming to the installation experience, if your device is already dev preview enabled, you can check for update by going to Settings—–>Check phone update and I first received a preparatory update, which makes it easy to update to Windows Phone 8.1. That preparatory update takes almost 10-15 minutes and after that either you or if you have enabled your device, it checks for next update. My device is currently updating with Windows Phone 8.1 and it takes good 30-40 minutes in installing completely. Remember,
- There should be enough battery or plug-in your device
- Better to be on WiFi
- No data is lost, so not to worry on that front