Absolutely! You can easily update your refill within the Bonnet app in Settings > My Pricing.
You will instantly be placed on your new refill amount and receive any extra electricity you pay for, but your card will not be charged until your next billing cycle. The pricing reconciliation will take place on your next refill renewal date, where you will be refunded what you haven't used from your old refill amount and a payment will be taken for your new refill amount (both on a pro-rata basis).
Please note that switching to a smaller Refill amount will not work if you have used more than the kWh that are included in the smaller Refill. For instance, if you have purchased 200kWh upfront and used 150kWh, the app will not allow you to switch to a 120kWh Refill amount until your next Refill renewal date.