To update income that you have already scheduled, go to your Activity tab (the second tab over) and select a future instance of your upcoming income event.
You'll be able to edit that income schedule directly on that screen, just be sure to tap the "Update Income" button at the bottom of the screen to save your changes.
To schedule additional income events, tap on a previous instance of that income (for example, your last paycheck) and tap on the "Repeats Every" field to create a new event.
Note: Sweep uses your income schedule to determine how much to automatically save for all your bills. Updating your income schedule will automatically re-adjust your bill buckets.