I'm not aware of a way to do this. You're asked what your activity level is when you add a user and it's used to determine your required number of calories. However, there does not seem to be a way to edit this.
One workaround is to create another user (File --> Manage Users --> + ). Create a new user with all your specs but with the modified activity level. Write down what the calorie range was set to for this new user. Then go back to your original account (File --> Manage users ... etc.) and manually modify the calorie range.