Click on Admin > Workflows > Ticket fields > Click on delete symbol near the custom field you have created. But once the field is deleted, you will lose the data pertaining to this field in all old tickets permanently.
It is not possible to delete a default ticket field as it is interlinked with other functionalities in the system. But, the field can be hidden from the customer's point of view and made non-mandatory for the agents while updating the tickets. Navigate to Admin > ticket fields > click on the concerned field and uncheck the boxes under agent as well as customer.