Add Textbox in Admin Panel for change data

I have some questions about custom fields.
Each custom fields for each post ,
I have a custom field with name: URL
I don't want to input data into URL field for each post because URL is unique and sometimes I want to change it but I want to change it once ( not go to each post for edit this field ) .

So,How can I create an input field like that in admin panel?

Thanks so much!