DataType Grid

// Get reference to content
var contentService = UmbracoContext.Current.Application.Services.ContentService;
var content = contentService.GetById(Model.CurrentPage.Id);

// Get original data
var data = new GridRowCollection(content.GetValue("dataTypeGrid").ToString());

// Change data
foreach (var row in data)
{
    var cell = row.GetCell("test");
    cell.Value = new Random().Next(0, 10).ToString();
}

content.SetValue("dataTypeGrid", data.ToString());

// Save content
var result = contentService.SaveAndPublish(content);

[DataTypeHandler(Priority = 123]
public class ContentPickerDataTypeHandler : BaseDataTypeHandler<PagePickerDataType>
{
    /// <summary>
    /// Method for customizing the way the <see cref="T">datatype</see> value is displayed in the grid.
    /// </summary>
    /// <remarks>Called when the grid displays the cell value for the specified <see cref="T">datatype</see>.</remarks>
    /// <param name="dataType">The <see cref="T">datatype</see> instance.</param>
    /// <returns>The display value.</returns>
    public override string GetDisplayValue(T dataType)
    {
        return base.GetDisplayValue(dataType);
    }

    /// <summary>
    /// Method for getting the typed backing object for the specified <typeparamref name="T">datatype</typeparamref>.
    /// </summary>
    /// <typeparam name="TBackingObjectType">The backing object type.</typeparam>
    /// <param name="dataType">The <typeparamref name="T">datatype</typeparamref> instance.</param>
    /// <returns>The backing object.</returns>
    /// <remarks>Called when the method <see cref="GridCell.GetPropertyValue" /> method is called on a <see cref="GridCell" />.</remarks>
    public override TBackingObjectType GetPropertyValue<TBackingObjectType>(T dataType)
    {
        return base.GetPropertyValue<TBackingObjectType>(dataType);
    }

    /// <summary>
    /// Method for getting the backing object for the specified <see cref="T">datatype</see>.
    /// </summary>
    /// <param name="dataType">The <see cref="T">datatype</see> instance.</param>
    /// <returns>The backing object.</returns>
    /// <remarks>Called when the method <see cref="GridCell.GetPropertyValue()" /> method is called on a <see cref="GridCell" />.</remarks>
    public override object GetPropertyValue(T dataType)
    {
        return base.GetPropertyValue(dataType);
    }

    /// <summary>
    /// Method for getting the control to use when validating the specified <see cref="IDataType" />.
    /// </summary>
    /// <param name="dataType">The <see cref="IDataType" /> instance.</param>
    /// <param name="editorControl">The <see cref="IDataType" /> editor control.</param>
    /// <returns>The control to validate.</returns>
    public override Control GetControlToValidate(T dataType, Control editorControl)
    {
        return base.GetControlToValidate(dataType, editorControl);
    }

    /// <summary>
    /// Method for performing special actions <b>before</b> creating the <see cref="IDataType" /> editor.
    /// </summary>
    /// <param name="dataType">The <see cref="IDataType" /> instance.</param>
    /// <param name="eventArgs">The <see cref="DataTypeLoadEventArgs"/> instance containing the event data.</param>
    /// <remarks>Called <b>before</b> the grid creates the editor controls for the specified <see cref="IDataType" />.</remarks>
    public override void Initialize(T dataType, DataTypeLoadEventArgs eventArgs)
    {
    }

    /// <summary>
    /// Method for performing special actions <b>after</b> the <see cref="IDataType" />
    /// <see cref="IDataEditor">editor</see> has been loaded.
    /// </summary>
    /// <param name="dataType">The <see cref="IDataType" /> instance.</param>
    /// <param name="eventArgs">The <see cref="DataTypeLoadEventArgs"/> instance containing the event data.</param>
    /// <remarks>Called <b>after</b> the grid creates the editor controls for the specified <see cref="IDataType" />.</remarks>
    public override void Configure(T dataType, DataTypeLoadEventArgs eventArgs)
    {
    }

    /// <summary>
    /// Method for executing special actions before saving the editor value to the database.
    /// </summary>
    /// <param name="dataType">The <see cref="T">datatype</see> instance.</param>
    /// <param name="eventArgs">The <see cref="DataTypeSaveEventArgs"/> instance containing the event data.</param>
    /// <remarks>Called when the grid is saved for the specified <see cref="T">datatype</see>.</remarks>
    public override void Save(T dataType, DataTypeSaveEventArgs eventArgs)
    {
        base.Save(dataType, eventArgs);
    }
} 

if (dataType.Data.Value != null)
{
    int id;

    if (int.TryParse(dataType.Data.Value.ToString(), out id))
    {
        var node = new Node(id);

        return string.Format("<a href='editContent.aspx?id={0}' title='Edit content'>Name: {1}<br/>Last updated: {2}</a>", node.Id, node.Name, node.UpdateDate);
    }

    return dataType.Data.Value.ToString();
}

return string.Empty;