How to modify a template

[rnbjunk]

Hello everybody, I'm trying to modify a custom template. I have downloaded the XML file from the admin control panel, but when I try to upload it back I get this error:

XML error : EntityRef: expecting ';' on line 5

I get this even if I try to upload the unmodified XML I just downloaded. How can I fix this?

I also tried another path (directly updating the template file) but I really have no clue where the template file itself resides. I found all the style sheets, but I don't know where else to look for the main file. Where can I find it?

Thanks for any help