You can upgrade your plan from settings tab.
Go to Settings Tab by clicking on the gear icon located on the top right of your page. On the left pane, click on Plan and Pricing. Now all the plans which are supported by Nurture will be displayed to you with all the description respectively. At the bottom of the current plan will be displayed Current and the plans which you can have Upgrade option at the bottom of the plans. When you click on upgrade button of any one plan, you will be redirected to payment page where you need to add your card details and make the payment and enjoy Nurturing.