Delete field

Delete field

Hello,
I want to erase a field. The problem is that when i go in Lead or Client in FIELDS LIST some CUSTOM FIELD are not available for change.
e.g. i want to erase MAILING STATE or MAILING STREET as a Text to replace it by the same with PICK UP LIST or MULTISELECT. No EDIT and no DELETE are possible.
Can you help please ?
Thank You