To modify the complication, you can do as following:
Hit the gear icon of the widget settings in the phone app, it should open the selector directly on your watch.
Just hit it while the watch face is displayed on your watch (not in settings or ambient mode).
Go to the settings of the watch face on your watch. Hold your finger on the screen until the settings screen shows up.
And then go in the widget category to modify the widget you want: select “complication external data” and it will open the complication chooser helper.
Scroll until you find the data you want to display on the watch face.