You can use this update sql query
update CMS_Document set DocumentPageTemplateConfiguration = '{"identifier":"PageTemplate.Page.TrackLostSavingsTemplate","properties":null}' where DocumentGUID=''
or you can update DocumentPageTemplateConfiguration column by using this
treeNode page = new DocumentQuery<TreeNode>()
.Path("/Articles/", PathTypeEnum.Children)
.OnSite("MySite")
.Culture("en-us")
.TopN(1)
.FirstOrDefault();
if (page != null)
{
// Updates the "DocumentName" and "ArticleTitle" fields of the page
page.SetValue("DocumentPageTemplateConfiguration ", "{"identifier":"Your Page Template identifier ","properties":null}");
page.Update();
}
}