PHP and XML load,edit,add

Hello everyone, I’m trying to load my xml file with php and have abilty to read that xml, edit existing entrys and to add new entrys to my xml. I have made this with few tutorials
as you can see i can add new xml entrys with this form and i can look existing entrys but i dont know how can i edit existing xml. In fact i want to be able to edit this tabe that you can see on this page.