Ok the code goes like this:

// JavaScript Document
function updatesum() {
document.sumario.total.value = (document.sumario.precio[]1.value -0) + (document.sumario.precio[]2.value -0) + (document.sumario.precio[]3.value -0) + (document.sumario.precio[]4.value -0) + (document.sumario.precio[]5.value -0) + (document.sumario.precio[]6.value -0) + (document.sumario.precio[]7.value -0);

I know that the problem here is the field name, but how do i make this work without having to change the names?

I can't change them because i have many other functions with those names already.