How can I modify the contents of a cell upon an event

I would like to modify the contents of a cell, triggered by an event tied to a cell.

I have a cell, CustomViewCell based on ViewCell. I have tried to override the OnLongPress method but does not seem to get hit.
I have setup a renderer for this, CustomViewCellRenderer in the android project:

     public class CustomViewCellRenderer: ViewCellRenderer
        {

            public CustomViewCellRenderer() { 

            }

            protected override Android.Views.View GetCellCore(Cell item, Android.Views.View convertView, ViewGroup parent, Context context)
            {

                var view =base.GetCellCore(item, convertView, parent, context);
                view.LongClick += 
                (object sender, Android.Views.View.LongClickEventArgs e)=>{

                    var cntx = this.Cell.BindingContext;
                    //cntx is now contains all ofthe data specific to that cell
                    //modify the cell content based on cntx
                };

                return view;
            }

This appears to work well, so far, so in the debugger, a breakpoint on var cntx = this.Cell.BindingContext; does get hit and cntx is the data that I want. How can I change the contents of the cell at that point (inside the event handler)? For example, how can I add a tick image to indicate that the data in the BindingContext for that cell was processed?

Cheers!