Update programmatically visitor details in Sitecore

You can use below approach to update visitor details programmatically.

if (Sitecore.Analytics.Tracker.Current.Contact != null)
     {
         var personalFacet = Tracker.Current.Contact.GetFacet<IContactPersonalInfo>("Personal");
         personalFacet.FirstName = "First Name";
         personalFacet.Surname = "Last Name";
         personalFacet.JobTitle = "Job Title";
     }

var phoneFacet = Tracker.Current.Contact.GetFacet<IContactPhoneNumbers>("Phone Numbers");
if (!phoneFacet.Entries.Contains("Phone Number"))
      {
           IPhoneNumber phoneNumber = phoneFacet.Entries.Create("Phone Number");
           phoneNumber.Number = "999999";
           phoneFacet.Preferred = "Phone Number";
      }

var contactEmail = Tracker.Current.Contact.GetFacet<Sitecore.Analytics.Model.Entities.IContactEmailAddresses>("Emails");

if (!contactEmail.Entries.Contains("Home"))
{
    contactEmail.Entries.Create("Home");
}

var email = contactEmail.Entries["Home"];
email.SmtpAddress = "[email protected]";;
contactEmail.Preferred = "Home";