I created a Group with a Header and a Footer. Then i tried to delete just the header, but it looks like there is no way to do so.
First : Creating my group header and footer
Secound : trying to delete my header
Result : it removed all rows in this group
Is there a way to delete only the header row without creating a new Tablix ?
You can do this in Row Group Section.
Click on marked, Here you will find Delete Group as well as what you want.