Since the earliest Apple Watch, you have been able to change your move goal (the red ring), but there was no way to change the exercise goal (the green ring) or the stand goal (the blue ring). Now, in watchOS 7, this is possible.
Open the Activity app on your watch, then scroll to the bottom with the digital crown. Tap Change Goals.
The first screen lets you change your move goal, counted in calories. Tap + or – to change it, then tap Next.
Next, you can change your exercise goal, in 5-minute increments, from 10 minutes to 60 minutes. Tap Next.
Finally, you can change your stand goal, from 6 to 12 hours. Tap OK.
It’s about time that Apple allows people to make these changes. There are many people who simply can’t do 30 minutes of exercise a day, and others who are frustrated that the watch only counts 30 minutes. So set your own goals, and close your rings more easily.