How can I change the data usage cycle for Mobile data?

The mobile data cycle (*Settings* -> *Data usage*) looks quite arbitrary. Since I have changed my data plan, now the new cycle *should* start at the first day of the month instead of the 6th. How can I change the begin of a data cycle? I cannot seem to find some edit button, though I vaguely remember seeing it at the *Data usage cycle* menu. Affected versions: Android 4.0, 4.2.2, 4.3.1.