To inactivate the custom field and then delete.
Step 1: Select the deactivate from the three dots of the custom field you want to deactivate.

Step 2: Select the inactive tab from the top left corner and open the list of the inactive custom field.

Step 3: Click on the three dots of the custom field you want to delete.

Step 4: Select Delete from the dropdown.

Please Note: The custom field that has already been used can not be deleted.