Hi,
You'll need to read up on JavaScript Webresources and their use in Form Events - msdn.microsoft.com/.../gg328261.aspx
To find the onchange code, you'll need to open the form editor - Customize Ribbon Tab ->Form->Click on the Service Field ->Change Properties->Events tab->Look at the functions being called 'Onchange' and find the code in the corresponding webresource.
hth,
Scott