How can you change the required intensity for a workout to be counted towards your exercise?

You can't adjust the intensity for a workout but you can do this;

You can choose "Other" workout type,With this workout you earn the credit equivalent of a "brisk walk" which means 1 minute credit for every 1 minute in the workout regardless of how fast or how slow you walk,this is particulary useful for anyone that can't walk at the brisk walking pace of 3.3 mph or are recovering from injury or are an older person who just can't reach the "brisk walking" speed.

So if you do 30 minutes in the "other" workout then you will close your green exercise ring.Active calories are also recorded and estimated at a rate equivalent to a "brisk walk" or based on data recorded by the heart rate sensor in the watch, whichever is higher.