How do I reset my red circle - activity circle?

Here's how to change your move goal:

1. Open the Activity app on your Apple Watch.
2. Go to your rings, then firmly press the screen.
3. Tap to increase or decrease the number of active calories for your daily Move goal.
4. When you're done, tap Update.

For some reason (with watchOS 5 and 6?), you can change the Move goal, but not your Exercise or Stand goals.  I believe this was changed in WatchOS 7 so that you can now change all the goals. See the "Change your activity goals" section in:

Use the Activity app on your Apple Watch - Apple Support

To restore a deleted workout type:

1. tap on workout app
2. then scroll down to the bottom and tap on "Add Workout"
3. finally tap on a workout type in the list

and it will be added back to your workout list.

To manually add a workout (and its data):

1. In the Health App on the iPhone, tap on the Health Data tab at the bottom
2. Then tap Activity and scroll down to Workouts and tap on it
3. In the the top right hand corner of the screen it says add data (or simply +)
4. Tap on that and you can enter a new workout and its data
5. When you are finished tap on add at the top right of the screen.

Be aware though that this only effects your 3 rings on the day in question.