Yes, within the time period specified by the project owner.
In order to do so, you must first cancel your first donation and then make another one.
Go to the “My transactions” section of your profile and click on the “cancel” icon on the transaction. The money will be automatically refunded for payments by credit card and Maestro. For other types of payments (PayPal / direct debit), the transaction will never happen.
Then, contribute to the project with the desired amount.