using (var server = new ServerManager())
{
    try
    {
        Site site = server.Sites[RoleEnvironment.CurrentRoleInstance.Id + "_Web"];
        string physicalPath = site.Applications["/"].VirtualDirectories["/"].PhysicalPath;
        string webConfigPath = Path.Combine(physicalPath, "web.config");

        var doc = System.Xml.Linq.XDocument.Load(webConfigPath);

        var redisCacheProviderSettings = doc.Descendants("sessionState").Single().Descendants("providers").Single().Descendants("add").Single();

        redisCacheProviderSettings.SetAttributeValue("host", RoleEnvironment.GetConfigurationSettingValue("SessionRedisCacheHost"));
        redisCacheProviderSettings.SetAttributeValue("accessKey", RoleEnvironment.GetConfigurationSettingValue("SessionRedisCacheAccessKey"));
        redisCacheProviderSettings.SetAttributeValue("ssl", "true");
        redisCacheProviderSettings.SetAttributeValue("throwOnError", "false");

        doc.Save(webConfigPath);
    }
    catch (Exception ex)
    {
        // Log error
    }
}