change category-field title

Hello
I want to change the title of (main) category (field) in a content submission form but I can't :
I tried many things but nothing works :
- write "category" in configuration / item form / "via CORE or 'CORE property' fields"
- configure core field "category" : in the field "category" and in the type of submission form
- add a new core property field
What's wrong ? Can you help me please ?

you need to create an override language

Ok thank you,
and do you know the name of the string or the value I have to change please ? (there are so many!...)

Hello
<?php echo JText::_( !$secondary_displayed || isset($all_tab_fields) ? 'FLEXICONTENT_CATEGORY' : 'FLEXI_MAIN_CATEGORY' ); ?>

BOTH in the backend language file of the component, so try to create language override for the backend and then also try adding to the frontend

if secodary cats are shown then it is:
FLEXI_MAIN_CATEGORY

if no secondary cats it is:
FLEXICONTENT_CATEGORY

it is an in-consistency because for other fields we use the label set in the field or in the type configuration