How to make data model changes to the existing .NET backend mobile service

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CodeFirst

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Hi friends,
I'm new to Entity Framework. In my current project, we have made use of Entity Framework: Code First Approach

Currently we have 1 table in the DB: Incident

Following entity for the Incident:

namespace IncidentReportingMobileService.DataObjects
{
  public class Incident : EntityData
    { 
        [Required]
        [StringLength(20)]
        public string UserName { get; set; }

        [StringLength(16)]
        public string ImmediateActions { get; set; }

        .
        .
        .
    }
}

Now I need to increase the length of the column ImmediateActions to 250 characters.

How can I achieve this? What is Entity Framework Code First Migrations?

Thanks in advance