If you voted from a Doodle account, make sure to log into that account to be able to edit/delete your vote.
On the poll page, move your mouse pointer over your name. A pencil icon (= Edit) appears. Click on the pencil icon. Now you can edit your participation. Furthermore, you now see a trash can icon next to your name, which you can click on in order to delete your participation completely.
When editing your participation, remember to click 'update' to save the changes you made.