Personalisation of Metrics on the watch screen can be achieved in several ways.
When you go into an activity, press the down button from the main screen and select Metrics and the Display. This will bring you to a sub-menu with Left and Right. Select Left to choose the metric you want to show on the left and Right to choose the metric you want to show on the right. The main screen is not pre-programmed. Once you start an activity you can press up and down to scroll through the available metrics which include, clock time, duration, distance, heart rate, calories, etc.
In addition, if using a Laps mode you will also see lap specific metrics.