// Your code to get an instance of the Tooling Service for
// an active Salesforce Session will probably differ.
ToolingServiceWrapper toolingService = salesforceSession.GetToolingService();

sObject[] customFields = toolingService.retrieve("Id, Metadata", "CustomField",
                                                 new string[] { "00N70000003KVWfEAO" });
Assert.AreNotEqual(0, customFields.Length);

CustomField cf = (CustomField)customFields[0];
if (cf.Metadata.formula == "Name")
{
    cf.Metadata.formula = "$Organization.Name";
}
else
{
    cf.Metadata.formula = "Name";
}
SaveResult[] saveResults = toolingService.update(new sObject[] { cf });
Assert.AreNotEqual(0, saveResults.Length);
SaveResult sr = saveResults[0];
Assert.IsTrue(sr.success);