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.
Connect on Ulule, then click on your username in the top right corner. Click on the contribution you want to cancel or on the more link if the the contribution is not one of the last you made.
On the detailed contribution page or form My contributions page, click on the Cancel icon on the right of the desired project.
Then, contribute to the project with the desired amount.