Exercise Intensity is calculated by percentage of maximum heart rate. (%HRmax, %MHR)
You can change the max heart in Settings > HEART RATE ZONES > Max Heart Rate.
The max heart rate is calculated with your age.
You can specify the formula to use.Please check references for understanding of these formulas.
You can set the max heart rate freely.
Turn off the "Use Estimate Value" switch and adjust it with - / +.