When you enter your exercise minutes, is that per day or per week?
Per day.
How do I reset the activity times or delete them?
Just tap on the ones you want to remove.
How do I switch between kJ and kcal?
Tap the big number at the very bottom (where it says total).
How do I set my carb limit to 20 grams?
Select the Expert Checkbox on the right and use the Net Carbs field.