You can use following example code to change property of web part (breadcrumbs) on some page template (cms.blog):
// get PageTemplateInfo according to PageTemplateCodeName
PageTemplateInfo pti = PageTemplateInfoProvider.GetPageTemplateInfo("CMS.Blog");
// get WebPartInstance according to WebPartID
WebPartInstance webPart = newInfo.GetWebPart("breadcrumbs");
// set web part property
if (webPart != null)
{
webPart.SetValue("Path", "/%");
}
// save changes to page template
CMS.PortalEngine.PageTemplateInfoProvider.SetPageTemplateInfo(newInfo);