<UpgradableAttribute> _
<SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel:=True)> _
<SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel:=True)> _
Public Class SPFarm
    Inherits SPPersistedUpgradableObject
    Implements IBackupRestore, IMigratable

Dim instance As SPFarm

[UpgradableAttribute] 
[SharePointPermissionAttribute(SecurityAction.InheritanceDemand, ObjectModel=true)] 
[SharePointPermissionAttribute(SecurityAction.LinkDemand, ObjectModel=true)] 
public class SPFarm : SPPersistedUpgradableObject, IBackupRestore, IMigratable

Dim farm As SPFarm = SPFarm.Local
Dim service As SPWebService = farm.Services.GetValue < SPWebService > ""

Dim webApp As SPWebApplication

For Each webApp In  service.WebApplications
    Dim job As SPJobDefinition

    For Each job In  webApp.JobDefinitions

        If job.Name = "MyCustomJobDefinitionName" Then
            j.Delete()
        End If
    Next job

    Dim newJob As New MyCustomJobDefinition("MyCustomJobDefinitionName", webApp)
    Dim schedule As SPSchedule = SPSchedule.FromString("every 5 minutes between 0 and 59")
    newJob.Schedule = schedule

    newJob.Update()
Next webApp

SPFarm farm = SPFarm.Local;
SPWebService service = farm.Services.GetValue<SPWebService>("");

foreach (SPWebApplication webApp in service.WebApplications)
{
    foreach (SPJobDefinition job in webApp.JobDefinitions)
    {
        if (job.Name == "MyCustomJobDefinitionName")
        {
            j.Delete();
        }
    }

    MyCustomJobDefinition newJob = new 
        MyCustomJobDefinition("MyCustomJobDefinitionName", webApp);
    SPSchedule schedule = SPSchedule.FromString("every 5 minutes between 0 and 59");
    newJob.Schedule = schedule;

    newJob.Update();
}