To delete the fields, you must first select the fields you wish to delete by clicking the Trash icon located on the right-hand side of the field you wish to delete. The field will then turn red as an indication that it has been selected for deletion. You can then select more fields before proceeding to click on the Delete Fields button to confirm deletion of all the selected fields.
See Edit Database fields for more options regarding database fields.
Deleting a field will remove all the data contained within. The data will not be recoverable.