Change my due date

Okay so I went to the doctor and had a transvaginal ultrasound and it estimated my baby's growth at 5wks 2 days. That way 5-29-15. But based on my last mp, glow has me calculated at 7wks 2days as of today and due on 1-17-2016. The doctors have me due at 1-21-2016. I'm leaning more towards the ultrasound estimate, although I know my due date will change at least 3 more times before  I actually go into labor, but how do I change Glows, due date on the app?