If you want to remove or delete any of your materials, you just need to browse to the material page and then click the Edit button at the top.
On the edit page, scroll down to the bottom and you will see two buttons: "Save" and "Delete". The delete button will ask for confirmation.
Please note that if, for example, you wrongly assigned a species or locality to a photo, you don't need to remove the record and upload it again. You can simply edit it, change the incorrect details and save.