How to update my weight in the Activity app
The Activity app uses the info stored in the Health app.

To change your weight:


Open the Watch app on the iPhone
On the My Watch tab, scroll down and tap on Health
Tap Edit in the top right corner
Scroll through the weights and select the correct one


You can also do this on the iPhone in the Health app under Body Measurements.

Once that is done, Activity will use the updated value.