Open Excel file on Office365 locally

Hi,

I'm writing a small data refresh utility that opens an XLSX file locally that is hosted on-line (Office365 SP), does a data refresh and then saves the file back to the SharePoint site.

The code I have so far is displayed below but I'm not sure how to navigate the SP doc library.

var targetSite = new Uri("https://<site_url>");
var login = "<user@tenant>.onmicrosoft.com";
var password = "<password>";
var securePassword = new SecureString();
foreach (char c in password)
{
securePassword.AppendChar(c);
}
var onlineCredentials = new SharePointOnlineCredentials(login, securePassword);

Can anyone help me complete the code or give a pointer to doing so.

Thanks

Lee