Windows 10 on lumia phones are eye catching. But my old lumia 625 didn't get any official update. Is their any way to update without getting any official update like we can do in android phones?
You can download and install the insider preview if you want to.
But your phone is not on the currently supported list for Insider preview. So you may not get future insider builds or updates. It's better to stay in Windows Phone 8.1
If you want to install Windows 10 insider preview anyway follow these steps: