Change the Monthly Gift Date
As there isn't a way to edit a subscription date once the initial date has been set (ex: change from the 15th of the month to the 1st), you'll need to first cancel your current subscription, then create a new monthly subscription on the date you want your subscription to be charged.
Part 1
Log in to your personal Donor Dashboard (found through any of the Harness fundraising pages or the email receipts you've received)
Select the person icon in the upper right-hand corner
Select "Monthly Gifts" from the drop-down menu
Identify your monthly subscription & select the 3 dots to the right of it
Select "Cancel Donation"
Part 2
*Do this part on the date you want the monthly gift to charge.
Select the person icon from the upper right-hand corner
Select "Monthly Gifts"
Under the section titled "Monthly Donations", set your donation amount
Select "Give Monthly"
Confirm your payment source (card on file or other), cover fee, whether/not to leave note or make gift in honor/memory
Select "Give"