Eto.Forms example of a DropDown that updates one of its item text using logic in the view model.

DropDownWithItemUpdate.cs

using System;
using Eto.Forms;
using Eto.Drawing;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Windows.Input;
using System.Linq;
using System.ComponentModel;
using System.Runtime.CompilerServices;

namespace DropDownWithItemUpdate
{
	public enum Resolution
	{
		Viewport, 
		Custom,
		R640X480, 
		R800X600, 
		R1024X768,
		R1280X960,
		R1600X1200,
		R2048X1536,
		R2560X1920,
		R3200X2400,
		R852X480, 
		R1280X720,
		R1366X768,
		R1920X1080
	}

	public class MyModel : INotifyPropertyChanged
	{
		public MyModel()
		{
			ResolutionNames = new ObservableCollection<ListItem>
			{
				new ListItem { Key = Resolution.Viewport.ToString(), Text = "Viewport" },
				new ListItem { Key = Resolution.Custom.ToString(), Text = "Custom" },
				new ListItem { Key = Resolution.R640X480.ToString(), Text = "640 x 480"},
				new ListItem { Key = Resolution.R800X600.ToString(), Text = "800 x 600"},
				new ListItem { Key = Resolution.R1024X768.ToString(), Text = "1024 x 768"},
				new ListItem { Key = Resolution.R1280X960.ToString(), Text = "1280 x 960"},
				new ListItem { Key = Resolution.R1600X1200.ToString(), Text = "1600 x 1200"},
				new ListItem { Key = Resolution.R2048X1536.ToString(), Text = "2048 x 1536"},
				new ListItem { Key = Resolution.R2560X1920.ToString(), Text = "2560 x 1920"},
				new ListItem { Key = Resolution.R3200X2400.ToString(), Text = "3200 x 2400"},
				new ListItem { Key = Resolution.R852X480.ToString(), Text = "852 x 480"},
				new ListItem { Key = Resolution.R1280X720.ToString(), Text = "1280 x 720"},
				new ListItem { Key = Resolution.R1366X768.ToString(), Text = "1366 x 768"},
				new ListItem { Key = Resolution.R1920X1080.ToString(), Text = "1920 x 1080"}
			};
		}

		Resolution resolution;
		public Resolution Resolution
		{
			get { return resolution; }
			set {
				if (resolution != value) {
					resolution = value;
					OnPropertyChanged ();
				}
			}
		}

		public ObservableCollection<ListItem> ResolutionNames { get; }

		public ICommand UpdateResolutionCommand
		{
			get {
				int count = 0;
				return new Command((sender, e) => {
					ResolutionNames[0] = new ListItem {
						Key = Resolution.Viewport.ToString(),
						Text ="Viewport " + count++
					};
					OnPropertyChanged(nameof(Resolution));
				});
			}
		}

		void OnPropertyChanged([CallerMemberName] string name = null)
		{
			PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
		}

		public event PropertyChangedEventHandler PropertyChanged;
	}

	public  class MainForm : Form
	{
		public MainForm ()
		{
			Title = "My Eto Form";
			ClientSize = new Size (400, 350);
			Menu = new MenuBar ();


			var updateItemButton = new Button { Text = "Update" };
			updateItemButton.BindDataContext (c => c.Command, (MyModel m) => m.UpdateResolutionCommand);

			var combo = new DropDown ();
			combo.BindDataContext (c => c.DataStore, (MyModel m) => m.ResolutionNames);
			combo.SelectedKeyBinding.BindDataContext (Binding.Property((MyModel m) => m.Resolution).EnumToString(), defaultContextValue: string.Empty);

			Content = new StackLayout {
				HorizontalContentAlignment = HorizontalAlignment.Stretch,
				Items = {
					"Combo",
					combo,
					updateItemButton
				}
			};

			DataContext = new MyModel ();
		}
	}
}