I forgot to start my exercise in Activity. How can I change the start time?

In the Health App > Workouts > select the date you want to change > tap on the "+" icon in the top left corner of the screen and add the correct workout: Activity Type, Kilocalories, Distance (km), Starts and Ends.

Then tap con "Show All Data" > it will show the new workout with a heart icon and the "wrong" workout with the icon of the Apple Watch or iPhone or the device which recorded it and tap Edit > and delete it. Then you will only see the manual record you just entered.

You will have to do on the iPhone Health app > Activity