Help a newbie?

Alexandria • TTC for 4+ years. Got BFP on 6/15/2015 and estimated due date is 2/6/2016!
I accidently hit the "I'm late button" so it moved my period to a later date. It won't let me put in my actual period because it's too close to the date it moved it to. It also won't let me delete the wrong date! How do I adjust this?