How can I edit a sequence?

Hi,

        I want to change some residues in my Seq object. How can I do
that? I looked at this page:

http://doc.bioperl.org/releases/bioperl-1.2/Bio/Seq.html

but I still haven't figured out how to do that. For normal strings, I can
do

substr($str, $index, 1, "r");

if I want to replace the character at $index by "r". Is there a similar
function in Seq?

Thanks
Yee Man