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.
Changing the starting day for Mobile data is indeed non-obvious. You need to enable the mobile data connection (2G/3G) before the options are available:

TURN OFF WIFI First.


Enable Mobile data.
Exit the Data usage settings app.
Tap the spinner below the Set mobile data limit checkbox.
Tap Change cycle....
Change Date of each month (I prefer the first day of the month, 1)
Hit Set.
Undo Mobile data if necessary (e.g. if on Wi-Fi).