<SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel:=True)> _
<SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel:=True)> _
Public Class SPField

Dim instance As SPField

[SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel=true)] 
[SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel=true)] 
public class SPField

Dim siteCollection As SPSite = SPContext.Current.Site
Dim sites As SPWebCollection = siteCollection.AllWebs("Site_Name").Webs
Dim site As SPWeb

For Each site In  sites

    Dim lists As SPListCollection = site.Lists
    Dim i As Integer

    For i = 0 To lists.Count - 1

        If lists(i).Title = "List_Name" Then

            Dim fieldChange As SPField = lists(i).Fields("Field_Name")

            fieldChange.DefaultValue = "Default_Value"
            fieldChange.Description = "Description"
            fieldChange.Title = "New_Field_Title"

            fieldChange.Update()

        End If

    Next i

Next site 

SPSite oSiteCollection = SPContext.Current.Site;
using (SPWeb oWebsite = oSiteCollection.AllWebs["Site_Name"])
{
    SPFieldCollection collFields = oWebsite.Lists["List_Name"].Fields;
    for (int intIndex = 0; intIndex < collFields.Count; intIndex++)
    {
        string[] strRefFields = collFields[intIndex].FieldReferences;

        if (strRefFields != null)
        {
            for (int intStrPos = 0; intStrPos < strRefFields.Length; 
                intStrPos++)
            {
                Label1.Text += collFields[intIndex].InternalName +
                " :: " + SPEncode.HtmlEncode(strRefFields[intStrPos]) 
                + "<BR>";
            }
        }
    }
}