Yes. At any time you can go to your dashboard and increase or decrease the amount of your savings plan. You can also change whether you want your plan to be weekly, biweekly, or monthly. Go to your Dashboard, then click "Change Plan or Bank Account."
Then you will be taken to the plan setup screen as shown below. Here you can change the amount and/or frequency of your plan. You can also change your banking information.
All changes will be updated on your next bank transfer.
For information on how that will affect your click here.