A donor can easily make changes to their Round-Up subscription by logging into their account via:
The "Welcome to Your Donor Dashboard" email they received when they first created their account with you (see example here)
One of your Harness giving pages (found either on your Web site or under your Giving Forms)
You can also edit the campaign designation of your donor's round-up gift or cancel their round-up account for them by...
From your Admin Dashboard, click Reports > Round-Ups
Search for the donor's name & click the 3 stacked dots on the right
Choose Edit Round-Ups or Cancel Round-Ups