Change Href Attribute With Two Dropdowns

I am in a problems here trying to generate link by two dropdown boxes. So, I wanted it to be like when user selects some option from dropdown one the href attribute changes, index.php?staticvar=something&dynvar1=something, that is something I have already managed to do:

function change_href(id)
{
if (id != '')
{
var dynvar1 = document.getElementById("s"+id).value;
document.getElementById(id).href = "index.php?staticvar="+id+"&dynvar1="+dynvar1;
return;
}
else
{
return false;
}
}
This works perfectly for me, but now I need to modify the href with another variable, so it would be needed to be like:
index.php?staticvar=something&dynvar1=something&dynvar2=something2

The problem is that I don't know how to make it the way that user can change their selections, for example if user first selects (from dropdown) dynvar1 to be "test" and dynvar2 to be "test3", but then changes hes mind and decides to re-select dynvar2 to be "test4". It would be needed to be like only the dynvar2 part of the href changes with proper value (not changing the dynvar1 or staticvar).

I managed to make it changeable but it was like this:
index.php?staticvar=something&dynvar1=something&dynvar2=123&dynvar2=123

So, it didn't update the dynvar2 part, but instead just added another dynvar2 variable.