how to edit expected period day?

Kassi • Mama of one girl 10202018❤️

this is the first time tracking my periods on this app. I don't know my cycle length because I've never really cared for tracking, so I put 28 day cycle. glow says today is the start of my period but it's not here, and I've gotten bfn for the past week so I'm assuming I just don't know my cycle lol. I try to say period is late and try to edit it but nothing changes. anyone know how?