The simplest way would be publishing some properties that change your
button. E.g. if you want to allow changing text of button2, add:

[Browsable(true), Category("MyControl")]
public string TextButton2 {
   get { return button2.Text; }
   set { button2.Text = value; }
}

The more difficult way would be writing own custom designer that allows
selecing the buttons within your control, adding a verb (menu item for
context menu for the control), or custom type in property grid that will
open a window with convenient editor for the buttons' properties.