Northern allowance automatically fills in the number of days, and I don't want the number of days that it fills in, how do I adjust them?

The only way to adjust the number of days for the Northern Residents Deduction is to "edit the dates" and create a shorter Period of Residence. 

Deductions Tab > Other Deductions > Northern Residents Deduction > click on the "Qualifies" button and make your change on the page that appears.