Days Between Cycles

When I first got the app it asked me how many days I have between my periods. Not like 5 or 6 days of being on my period but how many days between cycles like 28 or 34 days. I wrote a certain number but I was off so now whenever the app says something like “ur period starts in 2 days” i don’t actually get it in 2 days. Instead I get it a few days “late” (according 2 the app) so I end up never really knowing when I’m actually going 2 get my period. I know how to change cycles for when I’m actually on my period but that’s not what I need help with. I need help with how to edit the 28 or 34 days between cycles because when I first got the app and put that number in (30) the app said I could change it anytime I want but I don’t see how.