The first, and most important of all is that watchOS 3 improves up to seven times the speed of watchOS 2, which allows applications open instantly.
On the other hand, watchOS 3 includes a fully renovated Center Control and true multitasking, which lets you switch between applications quickly because they remain stored in the device memory.
Similarly, it has added a feature called Scribble, which can respond to any messages from Apple Watch drawing directly on the screen, which automatically become fully readable text.
Finally, physical activity also has a place in watchOS 3, as it is now possible to share our data with friends and family thanks to the "Activity Sharing" function.
How ever, to update you simply you have to access from your iPhone to the official app of Apple Watch and go to "General -> Software Update", but yes, I remind you that this must have iOS 10 minimum.