Howdy Philipkc,
Thanks for using the Apple Support Communities.
There are a couple of different ways in which Power Reserve can be enabled on your Apple Watch:
Turn on Power Reserve
If you need to, you can turn on Power Reserve. Press and hold the side button until you see the Power Off, Power Reserve, and Lock Device options. Then drag the slider to turn on Power Reserve.
You can also turn on Power Reserve using Glances. Swipe up on the watch face to open Glances, then swipe left or right to find the Battery Glance. Tap Power Reserve, then tap Proceed.
Using Power Reserve - Apple Support
Have a great weekend,
Alex H.