How to edit a Field name?

I wanted to know how can i edit a field name. I want to change the name ‘unit price’ to ‘carton price’ in sales invoice. I can’t edit it through form default option.

field names are not directly editable except for custom fields.
you can create a custom theme for this but that is beyond the scope of this forum.
you can hire any local programmer to do it for you if you personally do not have the necessary programming skills. the themes use liquid coding.